NETWORK MEDIA AND TOPOLOGIES

If you are wondering what media and topologies refer to, it all starts in the wiring closet. Media is the cabling, and topologies are the shapes that cabling and data transmissions take. Twenty percent (one-fifth) of the exam will contain questions that test your understanding of the following concepts of media and topologies.

So in this section we would presents the information needed to understand the common media used and the different types of topologies used on a network. This section covers the common media standards and the popular network components such as hubs, switches, bridges, routers, and gateways.

Wiring Tools Wire Crimper Map Testers Cable Testers Tone Generator firewall proxy

wire crimper

A wire crimper is a tool that you use to attach media connectors to the ends of cables. For instance, you use one type of wire crimper to attach RJ-45 connectors on Unshielded Twisted Pair (UTP) cable, and you use a different type of wire crimper to attach Bayonet Neill Concelman (BNCs) to coaxial cabling.

Wire Map Testers

A wire map tester is a device that is similar in principle to the tone generator and locator, except that it tests all the wire connections in a UTP cable at once. This device also consists of two parts, which you connect to the opposite ends of a cable. The unit at one end transmits signals over all the wires, which are detected by the unit at the other end. A wire map tester can detect transposed wires, opens, and shorts, just as a tone generator and locator can, but it does all the tests simultaneously and provides you with a simple readout telling you what's wrong

Multifunction Cable Testers

Multifunction cable testers are handheld devices, that perform a variety of tests on a cable connection and compare the results to standard values that have been programmed into the unit. The result is that these are devices that anyone can use. You simply connect the unit to the cable, press a button, and the device comes up with a list of pass or fail ratings for the individual tests. Multifunction cable testers can test any of the following:

• Length The most common method for determining the length of a cable is called time domain reflectometry (TDR), in which the tester transmits a signal over the cable and measures how long it takes for the signal's reflection to return. Using the nominal velocity of propagation (NVP) for the cable, which is the speed at which signals travel through the cable (supplied by the manufacturer) you can compute the length of the cable. This function also enables you to determine the location of a break in a cable.
• Attenuation By comparing the strength of a signal at the far end of a cable to its strength when transmitted, the tester determines the cable's attenuation (measured in decibels).
• Near end crosstalk (NEXT) Testing for near end crosstalk is a matter of transmitting a signal over one of a cable's wires and then detecting the strength of the signal that bleeds over into the other wires near the end of the cable where the transmitter is located.
• Power sum NEXT (PSNEXT) This is a measurement of the crosstalk generated when three of the four wire pairs are carrying signals at one time. This test is intended for networks using technologies like Gigabit Ethernet, which transmit signals over several wire pairs simultaneously.
• Equal level far end crosstalk (ELFEXT) This is a measurement of the crosstalk at the opposite end of the cable from the transmitter, corrected to account for the amount of attenuation in the connection.
• Power sum ELFEXT (PSELFEXT) This is a measurement of the crosstalk generated at the far end of the cable by three signal-carrying wire pairs, corrected for attenuation.
• Propagation delay This indicates the amount of time required for a signal to travel from one end of a cable to the other.
• Delay skew This is the difference between the lowest and the highest propagation delay measurements for the wires in a cable. Because the wire pairs inside a UTP cable are twisted at different rates, their relative lengths can differ, and the delay skew measurement quantifies that difference.
• Return loss This is a measurement of the accumulated signal reflection caused by variations in the cable's impedance along its length. These impedance variations are typically caused by untwisting too much of the wire pairs when making connections.

Tone Generator

One of the most basic ways to identify and test a cable connection is to use a tone generator and locator cable tester. The tone generator is a device that you connect to a cable at one end, and which transmits a signal over the cable. The tone locator is a separate device that has a probe capable of detecting the generator's signal, either by touching it to the conductor in the cable, or simply by touching it to the insulation on theoutside of the cable. When the locator detects the generator's signal, it emits an audible tone. You can use this type of device to test an entire cable, or to test the individual wire connections inside a UTP cable.

Tone generators are most commonly used to identify the cable belonging to a particular connection.

Example:
If you're performing an internal cable installation, and you forget to label one of your cables, you can connect the tone generator at the wall plate end and touch the probe to each of the cables at the patch panel end until you find the one that produces a tone. You can also use a tone generator and locator to test the individual wire connections inside a UTP cable.

• Connect the generator to a single wire or connector contact using alligator clips
• Then touch the locator to each wire or contact at the other end of the cable.

Using this method, you can test for any major wiring faults that affect internal UTP cable installations.
Example:

• If you fail to detect a signal on the contact to which you have the generator connected at the other end, you have an open circuit.
• If you detect a signal on the wrong contact, you have punched down the wires to the wrong contacts.
• If you detect a signal on two or more wires, you have a short.

Tone generator and locator Pros:

• Simple to use
• Most inexpensive type of cable tester
• Useful for troubleshooting a single cable connection.

Tone generator and locator Cons:

• Testing each of the wires in a UTP cable individually is time consuming
• You also need two people to use the equipment, one at the generator end and one at the locator end (unless you don't mind running back and forth from one end of your cable connections to the other)

Purpose, benefits and characteristics of using a firewall.

A firewall is a system or group of systems that enforces an access control policy between two networks. How this is accomplished varies widely, but in principle, the firewall can be thought of as a pair of mechanisms to either block or permit traffic. Some firewalls place a greater emphasis on blocking traffic, while others emphasize permitting traffic. Probably the most important thing to recognize about a firewall is that it implements an access control policy.

Firewall techniques:

• Packet filter looks at each packet entering or leaving the network and accepts or rejects it based on user-defined rules.
• Application gateway applies security mechanisms to specific applications, such as FTP and Telnet servers.
• Circuit-level gateway applies security mechanisms when a TCP or UDP connection is established. Once the connection has been made, packets can flow between the hosts without further checking.

Network layer firewalls
Network layer firewalls operate at a low level of the TCP/IP protocol stack as IP-packet filters, not allowing packets to pass through the firewall unless they match the rules. The firewall administrator may define the rules; or default built-in rules may apply. Modern firewalls can filter traffic based on many packet attributes like:

• source IP address
• source port
• destination IP address or port
• destination service like WWW or FTP

They can also filter based on protocols, TTL values, netblock of originator, domain name of the source, and many other attributes.

Application-layer firewalls
Application-layer firewalls work on the application level of the TCP/IP stack (i.e., all browser traffic, or all telnet or ftp traffic), and may intercept all packets traveling to or from an application. They block other packets without acknowledgement to the sender. Application firewalls can prevent all unwanted outside traffic from reaching protected machines.Firewalls can’t protect against attacks that don’t pass through it.

Types of firewalls

The term firewall is rather broad, because the features and effectiveness of any particular firewall vary greatly. However, firewalls in general can be classified into three basic forms, as outlined here:

• A dedicated hardware device
• A router with traffic filtering/firewall capabilities built in
• A software based system normally running on a server, PC, or MAC

Purpose, benefits and characteristics of using a proxy service.

A proxy server provides numerous advantages for connecting a local area network to the Internet. Acting as an Application-Layer Gateway, the proxy provides a strong defense from the outside world. Performing the duties of a firewall, however, is just one benefit of a proxy server. The proxy can also provide caching services to increase performance, logging services to track Internet use, tools to maximize the use of precious bandwidth, and content filtering to help keep unwanted data off the local network. The proxy can also utilize multiple connection types to easily provide redundancy and automatic failover in the event of a primary line failure.
The primary security features of Proxy Server are:

• It blocks inbound connections.
• LAN clients can initiate connections to Internet servers, but Internet clients cannot initiate connections to LAN servers.
• It can restrict outbound connections.

Rj-45 J Rj-11 USB MT-RJ Coaxial BNC LC Local Connector MT-RJ USB BNC connector AUI

RJ-11 (Registered Jack)

Standard telephone cable connectors, RJ-11 has 4 wires (and RJ-12 has 6 wires). RJ-11 is the acronym for Registered Jack-11, a four- or six-wire connector primarily used to connect telephone equipment.

RJ-11 Pin Signal Name 1 VCC (5 volts regulated) 2 Power Ground 3 One Wire Data 4 One Wire Ground

RJ-45 (Registered Jack)

The acronym for Registered Jack-45 is RJ-45. The RJ-45 connector is an eight-wire connector that is commonly used to connect computers to a local area network (LAN), particularly Ethernet LANs. Although they are slightly larger than the more commonly used RJ-11 connectors, RJ-45s can be used to connect some types of telephone equipment.

F-Type

The F connector is a type of RF connector commonly used for cable and universally for satellite television. They are also used for the cable TV connection in DOCSIS cable modems, usually with RG-6 tri-shield cable. The F connector is inexpensive, yet has good performance up to 1 GHz. One reason for its low cost is that it uses the center wire of the coaxial cable as the pin of the male connector. The male connector body is typically crimped onto the exposed outer braid. Femaleconnectors have a 3/8-32 thread. Most male connectors have a matching threaded connecting ring, though push-on versions are also available.

ST (Straight Tip) and SC (Subscriber Connector or Standard Connector)

Fiber network segments always require two fiber cables: one for transmitting data, and one for receiving. Each end of a fiber cable is fitted with a plug that can be inserted into a network adapter, hub, or switch. In the North America, most cables use a square SC connector (Subscriber Connector or Standard Connector) that slides and locks into place when inserted into a node or connected to another fiber cable, Europeans use a round ST connector (Straight Tip) instead.

SC connector

ST connector

Fiber LC (Local Connector)

These connectors are used for single-mode and multimode fiber-optic cables. FC connectors offer extremely precise positioning of the fiber-optic cable with respect to the transmitter's optical source emitter and the receiver's optical detector. FC connectors feature a position locatable notch and a threaded receptacle.

MT-RJ (Mechanical Transfer Registered Jack)

MT-RJ connectors are used with single-mode and multimode fiber-optic cables. The MT-RJ connectors are constructed with a plastic housing and provide for accurate alignment via their metal guide pins and plastic ferrules.

Used for Gigabit ethernet. To connect to modules with MT-RJinterfaces, use multimode fiber-optic cables.

 

USB (Universal Serial Bus)

Universal Serial Bus, or USB, is a computer standard designed to eliminate the guesswork in connecting peripherals to a PC. It is expected to replace serial and parallel ports. A single USB port can be used to connect up to 127 peripheral devices, such as mice, modems, keyboards, digital camera's, printers, scanners, MP3 players and many more. USB alsosupports Plug-and-Play installation and hot plugging.

• USB 1.1 standard supports data transfer rates of 12 Mbps.
• USB 2.0 (Also referred to as Hi-Speed USB) specification defines a new High-speed transfer rate of 480 Mb/sec.

USB 2.0 is fully compatible with USB 1.1 and uses the same cables and connectors. USB has with two connector types. The first is Type A (on the right), This connector connects to the PC's USB port. The Type B (on the left) connector and is for connecting to the relevant peripheral. Where as the type A connector is truly standard, the Type B connector could be changed in size etc. with individual peripherals meaning they require there own unique cables.

Cable media STP UTP SMF MMF Coaxial cable ThickNet RG-8 ThinNet RG-58

STP (Shielded Twisted Pair)

This cable has a conductive braided or foil casing for each pair and theoretically offers very good protection from interference and crosstalk. It was commonly used for token ring networks.

Shielded Twisted Pair is rarely used due to the fact that the potential performance increase over UTP is not worth the much greater cost of STP. STP, which is limited to 100-meter lengths (the same as UTP), is used in token ring networks and for IBM mainframe and minicomputer environments. There is no standard for it. Since token ring networks do not require STP, it is used less and less. These are few reasons for this:

• Higher cost due to greater complexity for the cabling and connectors
• Larger size and less flexibility of the cabling
• Longer installation time

UTP (Unshielded Twisted Pair)

UTP is the most commonly used type of networking cable. UTP cables are often called "ethernet cables" after Ethernet, the most common data networking standard that utilizes UTP cables, although not the most reliable.

In contrast to FTP and STP cabling, UTP cable is not surrounded by any shielding. It is the primary wire type for telephone usage and is very common for computer networking, especially in patch cables or temporary network connections due to the high flexibility of the cables.

Category 3

cable, commonly known as Cat-3, is an unshielded twisted pair (UTP) cable designed to reliably carry data up to 10 Mbit/s, with a possible bandwidth of 16 MHz. It is part of a family of copper cabling standards defined jointly by the Electronic Industries Alliance and the Telecommunications Industry Association. Category 3 was a popular cabling format among computer network administrators in the early 1990s, but has since been almost entirely replaced by the very similar Cat-5 standard, which offers higher top speeds.

Category 5

Cable, commonly known as Cat 5, is an unshielded twisted pair type cable designed for high signal integrity. The actual standard defines specific electrical properties of the wire, but it is most commonly known as being rated for its Ethernet capability of 100 Mbit/s. Its specific standard designation is EIA/TIA-568. Cat 5 cable typically has three twists per inch of each twisted pair of 24 gauge copper wires within the cable. The twisting of the cable reduces electrical interference and crosstalk.

Another important characteristic is that the wires are insulated with a plastic (FEP) that has low dispersion, that is, the dielectric constant of the plastic does not depend greatly on frequency. Special attention also has to be paid to minimizing impedance mismatches at connection points.

Cat 5 cables are often used in structured cabling for computer networks such as Fast Ethernet, although they are also used to carry many other signals such as basic voice services, token ring, and ATM (at up to 155 Mbit/s, over short distances).

Category 5e

cable is an enhanced version of Cat 5 for use with 1000BASE-T (gigabit) networks, or for long-distance 100 Base-T links (350 m, compared with 100 m for Cat 5). It must meet the EIA/TIA 568A-5 specification. Virtually all cables sold as Cat 5 are actually Cat 5e. The markings on the cable itself reveal the exact type.

Category 6

A cable standard for Gigabit Ethernet and other interconnect that is backward compatible with Category 5 cable, Cat-5e and Cat-3. Cat-6 features more stringent specifications for crosstalk and system noise. The cable standard is suitable for 10BASE-T / 100BASE-TX and 1000BASE-T (Gigabit Ethernet) connections.

The cable contains four twisted copper wire pairs, just like earlier copper cable standards, although each twisted pair is made up of slightly larger 23 gauge copper wire as opposed to Cat 5's 24 gauge wire. When used as a patch cable, Cat-6 is normally terminated in RJ-45 electrical connectors. If components of the various cable standards are intermixed, the performance of the signal path will be limited to that of the lowest category. The distance without losing data is 220 m.

Category 7

(CAT7), (ISO/IEC 11801:2002 category 7/class F), is a cable standard for Ultra Fast Ethernet and other interconnect technologies that can be made to be backwards compatible with traditional CAT5 and CAT6 Ethernet cable. CAT7 features even more stringent specifications for crosstalk and system noise than CAT6. To achieve this, shielding has been added for individual wire pairs and the cable as a whole.

The CAT7 cable standard has been created to allow 10-gigabit Ethernet over 100 m of copper cabling. The cable contains four twisted copper wire pairs, just like the earlier standards. CAT7 can be terminated in RJ-45 compatible GG45 electrical connectors which incorporate the RJ-45 standard, and a new type of connection to enable a smoother migration to the new standard. When combined with GG-45 connectors, CAT7 cable is rated for transmission frequencies of up to 600 MHz.

Coaxial cable

Coaxial cable is an electrical cable consisting of a round conducting wire, surrounded by an insulating spacer, surrounded by a cylindrical conducting sheath, and usually surrounded by a final insulating layer.

Most common use of coax (the short form of coaxial cable) today is in standard cable TV. If you have the chance to examine a cable, you will find that it has a fairly simple design. A copper conductor lies in the center of the cable, which is surrounded by insulation. A braided or mesh outer covering surrounds the insulation. This is also a conductor.

A PVC plastic jacket encases the covering The cable is designed to carry a high-frequency or broadband signal, as a high-frequency transmission line. Because the electromagnetic field carrying the signal exists (ideally) only in the space between the inner and outer conductors, it cannot interfere with or suffer interference from external electromagnetic fields.

ThickNet, or RG-8,

is older and one of the first types of coaxial cable used in networks. RG-8 is strung in a physical bus topology. Its thick shielding makes it fairly immune to noise but also very rigid and difficult to work with. RG-8 requires connectors, called vampire taps, that pierce through its thick outer shielding. Both ends of the bus must be terminated with a 50-ohm resistor; without both functioning resistors, the network will fail.

ThinNet or RG-58

cable is far more flexible than ThickNet and much easier to work with. RG-58 cabling is also strung as a physical bus. It is capable of connecting a maximum of 30 devices on up to a 185-meter length of cable. ThinNet is constructed like ThickNet, except that the central conductor and the insulation are much thinner. British Naval Connectors (BNCs) are crimped onto the cable for connectivity, and 50-ohm resistors are required at each end of the cable. They used to be common for implementing computer networks, in particular Ethernet, but twisted pair cables have replaced them in most applications.

SMF (Single Mode Fiber) optic cable

Single-mode optical fiber is an optical fiber in which only the lowest order bound mode can propagate at the wavelength of interest. Single mode fibers are best at retaining the fidelity of each light pulse over longer distances and exhibit no dispersion caused by multiple spatial modes; thus more information can be transmitted per unit time giving single mode fibers a higher bandwidth in comparison with multi-mode fibers. A typical single mode optical fiber has a core radius of 5-10 micrometers and a cladding radius of 120 micrometers. Currently, data rates of up to 10 Gigabits/second are possible at distances of over 60 km with commercially available transceivers. Equipment for Single mode fiber is more expensive than equipment for Multi-mode optical fiber, but the single mode fiber itself is usually cheaper in bulk.

MMF (Multimode Fiber) optic cable

Multi-mode optical fiber (multimode fiber or MM fiber) is a type of optical fiber mostly used for communication over shorter distances, e.g. within a building. It can carry 1 Gbit/s for typical building distances; the actual maximum speed depends upon the distance. It is easier to connect to than single-mode optical fiber, but its limit on speed. Multi-mode fiber has a larger center core than single-mode fiber, which allows it to support more than one propagation mode, or path within the fiber. The equipment used for communications over multi-mode optical fiber is less expensive than that for single-mode optical fiber. Typical transmission speeds/distances limits are 100 Mbit/s up to 2 km (100BASE-FX), 1 Gbit/s for distances up to 500-600 meters (1000BASE-LX, 1000BASE-SX), and 10 Gbit/s for distances up to 300 meters (10GBASE-SR).

COMPUTER HARDWARE

In this section we would cover some essential of computer hardware. We would start from very basic and goes to some advance topics. These topics would be very handy for you to locate the problem in computer hardware. You must have some basic skills in computer hardware even if you are preparing for network administrator job.

Computer hardware refers to objects that you can actually touch, like disks, disk drives, display screens, keyboards, printers, boards, and chips. In contrast, software is untouchable. Software exists as ideas, application, concepts, and symbols, but it has no substance. A combination of hardware and software forms a usable computing system. Computer hardware is the collection of physical elements that comprise a computer system. Computer hardware refers to the physical parts or components of computer such as monitor, keyboard, hard disk, mouse, etc.

Computer hardware review and definition

Network administrator must be familiar with basic computer hardware operations. He should have adequate knowledge of computer hardware to perform day by day task. So in this article we would cover some basic computer hardware terms. This could also be helpful for interview.

What is a Computer?

• An electronic machine
• that can be programmed to
• accept data (input), and
• process it into
• useful information (output).
• Kept in secondary storage (storage) for safekeeping or later use.
• The processing of input into output is directed by the software, but performed by the hardware.

• The System Unit

  The System Unit houses the central processing unit, memory modules, expansion slots, and electronic circuitry as well as expansion cards that are all attached to the motherboard; along with disk drives, a fan or fans to keep it cool, and the powersupply.
  All other devices (monitor, keyboard, mouse, etc., are linked either directly or indirectly into the system unit.)

  The Motherboard

  The motherboard is the main circuit board of a computer. It contains the central processing unit (CPU), the Basic Input/Output System (BIOS), memory, mass storage interfaces, serial and parallel ports, expansion slots, and all the controllers for standard peripheral devices like the keyboard, disk drive and display screen.
  The chipset is a critical part of any computer, because it plays a big role in determining what sorts of features the computer 

  Hardware components

  Input devices -
  accept data or commands in a form useable by computers
  Output devices
  display the processed information - printers, monitors, speakers.
  Processing devices
  In system unit and are comprised of circuitry.
  Storage devices -
  Drives read from and write to storage media (the physical material that can store data and programs).
  Communication devices
  provide connections between computers and communication networks, allowing for exchange of information and data with other computers via transmission media such as cables, telephone lines, and satellites

  Basic computer hardware terminology

  • 
  In this article we would cover basic of computer hardware. Being a system administrator you should familiar with computer hardware terminology.

  The Motherboard

  The motherboard is the main circuit board of a computer. It contains the central processing unit (CPU), the Basic Input/Output System (BIOS), memory, mass storage interfaces, serial and parallel ports, expansion slots, and all the controllers for standard peripheral devices like the keyboard, disk drive and display screen.
  The chipset and other motherboard circuitry are the "smarts" of the motherboard. Their job is to direct traffic and control the flow of information inside the computer.
  The chipset is a critical part of any computer, because it plays a big role in determining what sorts of features the computer can support.
  

  BIOS

  • BIOS stands for Basic Input/Output System.
  • lowest-level software in the computer
  • Acts as an interface between the hardware (especially the chipset and processor) and the operating system.
  • The BIOS provides access to the system hardware and enables the creation of the higher-level operating systems that you use to run your applications.
  • The BIOS is also responsible for allowing you to control your computer's hardware settings, for booting up the machine when you turn on the power or hit the reset button, and various other system functions.

  • ROM: Read Only Memory

    • ROM is nonvolatile. ROM chips contain permanently written data, called firmware (your BIOS lives here).
    • ROM contains the programs that direct the computer to load the operating system and related files when the computer is powered on.
    • ROM chips are usually recorded when they are manufactured.
    PROM -Programmable Read Only memory chip cannot be changed to update or revise the program inside
    EPROM Erasable Programmable Read Only memory Data can be erased and chip can be reused Can be erased by shininghigh intensity UV light through the window
    EEPROM Electrical Erasable Programmable Read Only memory under high voltage
    FROM -Flash ROM is reprogrammable memory using normal voltage inside the PC- You can upgrade the logic capabilitiesby simply downloading new software. This saves the expense of replacing circuit boards and chips.

    Processing Devices

    

    Cache

    Pronounced cash.
    It is a small, high-speed memory area that is placed between the processor and the system memory.
    The value of the cache is that it is much faster than normal system memory.
    The most frequently used instructions are kept in cache memory so that the CPU can look in there first - allows the CPU to run faster because it doesn't have to take time to swap instructions in and out of main memory.
    Large, complex programs such as complex spreadsheets or database management programs benefit the most from having a cache memory available. Pentium II processors generally come with at least 512 KB of cache memory.

    Random Access Memory (RAM)

    • RAM is Primary Storage, also called internal storage.
    • Serves as computers workspace, storing all or part of the program that is being executed, as well as data being used by the program.
    • RAM provides instructions and data to the CPU.
    • These instructions/data are coded in bytes.
    • Each byte is placed in a precise location in memory, called an address.
    • To access data or instructions in memory, the computer references the addresses containing the bytes.
    • The amount of memory available is therefore measured in bytes
    
    • RAM chips consist of millions of switches that are sensitive to changes in electric current.
    • RAM chips are typically packaged on small circuit boards called memory modules, which are inserted into special slots on the motherboard.
    • RAM is Volatile storage: Power goes, data goes!
    • Data/instructions are copied into memory as needed.
    • Not enough memory or corruption of data/instructions in memory can cause crash.
    • On booting, operating system files are loaded from a storage device (the hard disk, usually) into RAM, and they remain there as long as your computer is running.
    • RAM contents changes as programs are executed.
    • RAM chips consist of millions of switches that are sensitive to changes in electric current.
    • RAM chips are typically packaged on small circuit boards called memory modules, which are inserted into special slots on the motherboard.
    • On booting, operating system files are loaded from a storage device (the hard disk, usually) into RAM, and they remain there as long as your computer is running.
    • RAM contents changes as programs are executed.
    • The amount of RAM needed depends on the types of applications you intend to run on the computer. S/w indicate the minimum amount of RAM required to run.
    Two basic types of RAM are Dynamic RAM (DRAM), and Static RAM (SRAM).
    Most computers today use DRAM, which are also of two types:
    • SDRAM Synchronous Dynamic RAM runs at the same pace as the system clock runs
    • DDR SDRAM DDR stands for Double Data Rate - runs at double the pace the system clock runs - available in speeds from 266 MHZ upto 600MHZ
    • DDR2 SDRAM runs at four times the pace the system clock runs - available in speeds from 400 MHZ upto 800MHZ
    Most desktops and notebooks use one of the three most popular types of synchronous dynamic random access memory (SDRAM) for the main system memory. Single data rate (SDR) SDRAM is the older type of memory, commonly used in computers prior to 2002. Double data rate (DDR) SDRAM hit the mainstream computer market around 2002, and DDR2-based systems hit the market in mid-2004.
    DDR SDRAM is a straightforward evolution from SDR SDRAM. The big difference between DDR SDRAM and SDR SDRAM is that DDR reads data on both the rising and falling edges of the clock signal, so the DDR module can transfer data twice as fast as SDR SDRAM.
    While DDR has a limited clock rate, the evolutionary changes to DDR architecture enable DDR2 to achieve speeds beyond of DDR, delivering bandwidth of 5.3 GB per second and beyond! Because DDR2 is able to operate with faster bus speeds, your memory doesn't hold back the performance of your processor.
    Generally speaking, motherboards are built to support only one type of memory. You cannot mix and match SDRAM, DDR, or DDR2 memory on the same motherboard in any system. They will not function and will not even fit in the same.

    Why is RAM so important?

    Aside from the processor, the two most important factors affecting a PC's performance are RAM and hard disk capacity.
    Hard disks are typically huge, so the primary limiting factor is the amount of installed RAM.
    Without enough RAM, the operating system must swap out storage space with the hard disk. The OS creates a Paging File (swap file) to supplement RAM (workspace). This is Virtual Memory.
    Virtual memory is inherently slow! RAM speed can typically be 120,000 times FASTER than the hard disk so the less you must rely on virtual memory (swapping files between RAM and hard disk), the faster your system will perform.

    Microprocessor

    • Heart and brain of the PC
    • One electrical circuit in control of another
    • Successive generation of processors
    • 80286,80386,80486 -32 bit interface
    • Pentium family P1, P2, P3, P4 64 bit interface
    • Dual-core technology is like having two processors - A dual core processor is a CPU with two separate cores residing on the same chip

    Definitions computers hardware basic Coding Schemes Computer Peripherals

    • 
    
    
    In this article we would discuss about coding schemes used in computer, definition of computer. Later we would cover measurement unit used in computer storages.

    What is a Computer?

    • An electronic machine
    • that can be programmed to
    • accept data (input), and
    • process it into
    • useful information (output).
    • Kept in secondary storage (storage) for safekeeping or later use.
    • The processing of input into output is directed by the software, but performed by the hardware.
    

    Bits & Bytes

    Computers are devices powered by electricity, which has two discrete states: On or Off.
    Two digits represent these states:
    
    • To be processed, all data in a computer system (words, symbols, pictures, videos, sounds) must be reduced to a string of binary digits.
    • A binary digit 1 or 0 is called a bit,
    • Eight bits grouped together as a unit are called a byte, which provides enough combinations of 0s and 1s to represent 256 individual characters, including numbers, upper and lower case alphabet letters, punctuation marks and other characters

    Name	Abb	Approx. Bytes	Exact Bytes	Approx. Pages of Text
    Byte	B	One	1	One character
    Kilobyte	KB (or K)	One thousand	1,024	One-half page
    Megabyte	MB	One million	1,048,576	500 pages
    Gigabyte	GB	One billion	1,073,741,824	500,000 pages
    Terabyte	TB	One trillion	1,099,511,627,776	500,000,000 pages

    Coding Schemes

    Define the patterns of bytes
    Coding schemes, such as ASCII, EBCDIC, and Unicode, provide the means to interact with a computer
    When a letter is pressed on a keyboard, the electronic signals are converted into binary form and stored into memory.
    The computer processes the data as bytes of information and converts them to the letters we see on the monitor screen or on a printed page.
    

    Terminology

    • Hardware (H/w) All machinery & Equipments Computer & Peripherals
    • Peripherals Any piece of hardware connected to the PC
    • Software (S/w) programs- tells the Computer how to perform a task
    • Systems Software (S/w) For managing internal activities & run applications s/w Interpreter bet S/w & H/w
    • Application Software (S/w) - to perform a specific task Custom or Packaged

    Computer hardware components Input Devices Output Devices

    • 
    I have given a brief over view about computer hardware terminology in our last article. Now in this article I would discuss about hardware components of computer.

    Hardware components

    Input devices -
    accept data or commands in a form useable by computers
    Output devices
    display the processed information - printers, monitors, speakers.

    Processing devices

    in system unit and are comprised of circuitry.

    Storage devices -

    Drives read from and write to storage media (the physical material that can store data and programs).

    Communication devices

    provide connections between computers and communication networks, allowing for exchange of information and data with other computers via transmission media such as cables, telephone lines, and satellites
    

    Input Devices

    • Keyboards
    • Pointing Devices mouse, trackballs, joysticks, touchpads and light pens
    • Source Entry devices Scanners, Audio input devices, video input devices, digital cameras

    • Output Devices

      Softcopy

      Monitor /Display Screens CRT and Flat Panel (LCD), EL and gas plasma
      Monitor /Display Screen clarity
      Resolution 
      refers to the number of dots displayed in the X (across) and Y (down) co-ordinates. 
      expressed in terms of horizontal pixels X vertical pixels. 
      Typical screens are capable of displaying 640x480 dots
      Dot Pitch 
      measurement of how close together the pixels, or phosphor dots, are that make up an image. 
      The smaller the dot pitch, the crisper the image,0.31 or less provides a sharp image, especially when displaying text.
      Refresh rate 
      the vertical frequency, or the rate at which each pixel on a screen is re-drawn. A low refresh rate results in an image that flickers, resulting in eye-strain. 
      A refresh rate of 60Hz means the images is redrawn 60 times a second. Typical refresh rates are 60Hz, 72Hz and 75Hz.

      Video Display Adapters

      Display graphics - Visual output from your system. 
      Works between the system's processor and monitor 
      Relays the information received from the programs and applications running on the system to the monitor
      VDAs come with their own memory chips (RAM or VRAM for video RAM) which determines how fast the card processes images, the resolution, and how many colours it can display. 
      VDA embody certain standards. 
      Todays PCs commonly use VGA and SVGA standards

      Hardcopy Output : Printers

      Impact Printers

      The general features of impact printers are uses force by applying hammer pins to strike the paper
      • slow speed
      • prints on most paper types
      • transparencies not supported
      • multiple copies may be printed at once
      Advantages : Less expensive, Fast (some types) , Can make multiple copies with multipart paper
      Disadvantages : Noisy! Print quality lower in some types. Poor graphics or none at all.

      Dot-Matrix and Daisy-Wheel.

      Dot matrix printers form characters using row(s) of pins, 9, 18, or 24 which impact the ribbon on top of the paper.
      Daisy wheel printers use a spoked wheel with characters placed at the end of each spoke. A print hammer is used to strike the desired character onto the ink ribbon and then the paper.

      Hardcopy Output : Printers

      Non Impact Printers

      General features print head does not make contact with the paper
      • higher speed in characters per second is possible
      • prints on most paper types but better quality obtained with better paper
      • transparencies usually supported
      • Uses ink spray or toner powder
      • Offer superior quality and greater options (in terms of the number of fonts and quality of graphic pictures)
      Disadvantages : more expensive.
      The three main types of non-impact printers are laserjet, inkjet and thermal

      Characters of printers

      Speed: The speed of a printer is measured in: cps= characters per second, lpm= lines per minute ppm= pages per minute The faster the printing, the more expensive the printer.
      Resolution: A more numerical measure of print quality is printer resolution. Measured in dots per inch (dpi), this determines how smooth a diagonal line the printer can produce.

      Cable connection:

      Serial Cables- send data only 1 bit at a time- Distance from PC 1000 ft
      Parallel Cables- send data 8 bits at a time. Distance from PC 50 ft.- Most popular - USB cable which has a maximum data transfer speed of 12 megabits/s (1.5 MBYTES/s).

      Examples of computer hardware System Unit Types of Ports Types of Connectors

      The System Unit

      The System Unit houses the central processing unit, memory modules, expansion slots, and electronic circuitry as well as expansion cards that are all attached to the motherboard; along with disk drives, a fan or fans to keep it cool, and the power supply.

      All other devices (monitor, keyboard, mouse, etc., are linked either directly or indirectly into the system unit.

      

      Front of the System Unit

      Drives are housed in drive bays which are accessed at the front of the case.

      Internal drives, such as the hard disk drive, are installed in internal bays that are not typically as accessible as the external drives pictured here.

      

      System Unit cases come in a huge array of types and styles, depending upon hardware needs.

      Types of Ports

      

      Serial ports

      transmit data one bit at a time, like the picture on the left illustrates.

      Parallel ports

      transmit more than one byte at a time.

      These types of port designs are based on whether or not fast data transmission rates are required by the device or not.

      Most computers come with basic types of ports (serial, parallel, keyboard, mouse, and USB); and expansion cards allow you to expand the available types needed by specific devices.

      Different Types of Connectors

      

      Understanding the differences among connector types is useful and important, as the cable required to attach a device to your computer is specific to its connector, not to mention the port on the computer.

      Non-Volatile Storage Devices

      Disk drives

      • Internal & External
      • Hard drives
      • Removable disk drives
      • Floppy disks (1.4 MB)
      • ZIP disks (100/250 MB)
      • CD-ROM (700MB), DVD-ROM (~5GB/side)
      • read only (-ROM), write once (-R), re-writeable (-RW)
      • Combination drive
      • CD-RW/DVD-ROM, CD-RW/DVD-R

      Many other forms
      Memory Stick, MultiMediaCard, CompactFlash, and SmartMedia

      
      

      Networking Devices Hub Switch Router Modem Bridges Brouters Gateways

      HUB

      Networks using a Star topology require a central point for the devices to connect. Originally this device was called aconcentrator since it consolidated the cable runs from all network devices. The basic form of concentrator is the hub.

       

      As shown in Figure; the hub is a hardware device that contains multiple, independent ports that match the cable type of the network. Most common hubs interconnect Category 3 or 5 twisted-pair cable with RJ-45 ends, although Coax BNC and Fiber Optic BNC hubs also exist. The hub is considered the least common denominator in device concentrators. Hubs offer an inexpensive option for transporting data between devices, but hubs don't offer any form of intelligence. Hubs can be active or passive.

      An active hub strengthens and regenerates the incoming signals before sending the data on to its destination.

      Passive hubs do nothing with the signal.

      Ethernet Hubs

      An Ethernet hub is also called a multiport repeater. A repeater is a device that amplifies a signal as it passes through it, to counteract the effects of attenuation. If, for example, you have a thin Ethernet network with a cable segment longer than the prescribed maximum of 185 meters, you can install a repeater at some point in the segment to strengthen the signals and increase the maximum segment length. This type of repeater only has two BNC connectors, and is rarely seen these days.

       
      8 Port mini Ethernet Hub

      The hubs used on UTP Ethernet networks are repeaters as well, but they can have many RJ45 ports instead of just two BNC connectors. When data enters the hub through any of its ports, the hub amplifies the signal and transmits it out through all of the other ports. This enables a star network to have a shared medium, even though each computer has its own separate cable. The hub relays every packet transmitted by any computer on the network to all of the other computers, and also amplifies the signals.

      The maximum segment length for a UTP cable on an Ethernet network is 100 meters. A segment is defined as the distance between two communicating computers. However, because the hub also functions as a repeater, each of the cables connecting a computer to a hub port can be up to 100 meters long, allowing a segment length of up to 200 meters when one hub is inserted in the network.

      Multistation Access Unit

      

      A Multistation Access Unit (MAU) is a special type of hub used for token ring networks. The word "hub" is used most often in relation to Ethernet networks, and MAU only refers to token ring networks. On the outside, the MAU looks like a hub. It connects to multiple network devices, each with a separate cable.

      Unlike a hub that uses a logical bus topology over a physical star, the MAU uses a logical ring topology over a physical star.

      

      When the MAU detects a problem with a connection, the ring will beacon. Because it uses a physical star topology, the MAU can easily detect which port the problem exists on and close the port, or "wrap" it. The MAU does actively regenerate signals as it transmits data around the ring.

      Switches

      

      Switches are a special type of hub that offers an additional layer of intelligence to basic, physical-layer repeater hubs. A switch must be able to read the MAC address of each frame it receives. This information allows switches to repeat incoming data frames only to the computer or computers to which a frame is addressed. This speeds up the network and reduces congestion.

      

      Switches operate at both the physical layer and the data link layer of the OSI Model.

      Bridges

      A bridge is used to join two network segments together, it allows computers on either segment to access resources on the other. They can also be used to divide large networks into smaller segments. Bridges have all the features of repeaters, but can have more nodes, and since the network is divided, there is fewer computers competing for resources on each segment thus improving network performance.

      

      Bridges can also connect networks that run at different speeds, different topologies, or different protocols. But they cannot, join an Ethernet segment with a Token Ring segment, because these use different networking standards. Bridges operate at both the Physical Layer and the MAC sublayer of the Data Link layer. Bridges read the MAC header of each frame to determine on which side of the bridge the destination device is located, the bridge then repeats the transmission to the segment where the device is located.

      Routers

      Routers Are networking devices used to extend or segment networks by forwarding packets from one logical network to another. Routers are most often used in large internetworks that use the TCP/IP protocol suite and for connecting TCP/IP hosts and local area networks (LANs) to the Internet using dedicated leased lines.

      

      Routers work at the network layer (layer 3) of the Open Systems Interconnection (OSI) reference model for networking to move packets between networks using their logical addresses (which, in the case of TCP/IP, are the IP addresses of destination hosts on the network). Because routers operate at a higher OSI level than bridges do, they have better packet-routing and filtering capabilities and greater processing power, which results in routers costing more than bridges.

      

      Routing tables

      Routers contain internal tables of information called routing tables that keep track of all known network addresses and possible paths throughout the internetwork, along with cost of reaching each network. Routers route packets based on the available paths and their costs, thus taking advantage of redundant paths that can exist in a mesh topology network.

      Because routers use destination network addresses of packets, they work only if the configured network protocol is a routable protocol such as TCP/IP or IPX/SPX. This is different from bridges, which are protocol independent. The routing tables are the heart of a router; without them, there's no way for the router to know where to send the packets it receives.

      Unlike bridges and switches, routers cannot compile routing tables from the information in the data packets they process. This is because the routing table contains more detailed information than is found in a data packet, and also because the router needs the information in the table to process the first packets it receives after being activated. A router can't forward a packet to all possible destinations in the way that a bridge can.

      • Static routers: These must have their routing tables configured manually with all network addresses and paths in the internetwork.
      • Dynamic routers: These automatically create their routing tables by listening to network traffic.
      • Routing tables are the means by which a router selects the fastest or nearest path to the next "hop" on the way to a data packet's final destination. This process is done through the use of routing metrics.
      • Routing metrics which are the means of determining how much distance or time a packet will require to reach the final destination. Routing metrics are provided in different forms.
      • hop is simply a router that the packet must travel through.
      • Ticks measure the time it takes to traverse a link. Each tick is 1/18 of a second. When the router selects a route based on tick and hop metrics, it chooses the one with the lowest number of ticks first.

      You can use routers, to segment a large network, and to connect local area segments to a single network backbone that uses a different physical layer and data link layer standard. They can also be used to connect LAN's to a WAN's.

      Brouters

      Brouters are a combination of router and bridge. This is a special type of equipment used for networks that can be either bridged or routed, based on the protocols being forwarded. Brouters are complex, fairly expensive pieces of equipment and as such are rarely used.

      

      A Brouter transmits two types of traffic at the exact same time: bridged traffic and routed traffic. For bridged traffic, the Brouter handles the traffic the same way a bridge or switch would, forwarding data based on the physical address of the packet. This makes the bridged traffic fairly fast, but slower than if it were sent directly through a bridge because the Brouter has to determine whether the data packet should be bridged or routed.

      Gateways

      A gateway is a device used to connect networks using different protocols. Gateways operate at the network layer of the OSI model. In order to communicate with a host on another network, an IP host must be configured with a route to the destination network. If a configuration route is not found, the host uses the gateway (default IP router) to transmit the traffic to the destination host. The default t gateway is where the IP sends packets that are destined for remote networks. If no default gateway is specified, communication is limited to the local network. Gateways receive data from a network using one type of protocol stack, removes that protocol stack and repackages it with the protocol stack that the other network can use.

      Examples

      • E-mail gateways-for example, a gateway that receives Simple Mail Transfer Protocol (SMTP) e-mail, translates it into a standard X.400 format, and forwards it to its destination
      • Gateway Service for NetWare (GSNW), which enables a machine running Microsoft Windows NT Server or Windows Server to be a gateway for Windows clients so that they can access file and print resources on a NetWare server
      • Gateways between a Systems Network Architecture (SNA) host and computers on a TCP/IP network, such as the one provided by Microsoft SNA Server
      • A packet assembler/disassembler (PAD) that provides connectivity between a local area network (LAN) and an X.25 packet-switching network

      CSU / DSU (Channel Service Unit / Data Service Unit)

      A CSU/DSU is a device that combines the functionality of a channel service unit (CSU) and a data service unit (DSU). These devices are used to connect a LAN to a WAN, and they take care of all the translation required to convert a data stream between these two methods of communication.

      

      A DSU provides all the handshaking and error correction required to maintain a connection across a wide area link, similar to a modem. The DSU will accept a serial data stream from a device on the LAN and translate this into a useable data stream for the digital WAN network. It will also take care of converting any inbound data streams from the WAN back to a serial communication.

      A CSU is similar to a DSU except it does not have the ability to provide handshaking or error correction. It is strictly an interface between the LAN and the WAN and relies on some other device to provide handshaking and error correction.

      NICs (Network Interface Card)

      Network Interface Card, or NIC is a hardware card installed in a computer so it can communicate on a network. The network adapter provides one or more ports for the network cable to connect to, and it transmits and receives data onto the network cable.

      Wireless Lan card
      

      Every networked computer must also have a network adapter driver, which controls the network adapter. Each network adapter driver is configured to run with a certain type of network adapter.

      Network card
      

      Network Interface Adapter Functions 
      Network interface adapters perform a variety of functions that are crucial to getting data to and from the computer over the network.

      These functions are as follows:

      Data encapsulation
      The network interface adapter and its driver are responsible for building the frame around the data generated by the network layer protocol, in preparation for transmission. The network interface adapter also reads the contents of incoming frames and passes the data to the appropriate network layer protocol.

      Signal encoding and decoding
      The network interface adapter implements the physical layer encoding scheme that converts the binary data generated by the network layer-now encapsulated in the frame-into electrical voltages, light pulses, or whatever other signal type the network medium uses, and converts received signals to binary data for use by the network layer.

      transmission and reception
      The primary function of the network interface adapter is to generate and transmit signals of the appropriate type over the network and to receive incoming signals. The nature of the signals depends on the network medium and the data-link layer protocol. On a typical LAN, every computer receives all of the packets transmitted over the network, and the network interface adapter examines the destination address in each packet, to see if it is intended for that computer. If so, the network interface adapter passes the packet to the computer for processing by the next layer in the protocol stack; if not, the network interface adapter discards the packet.

      Data buffering 
      Network interface adapters transmit and receive data one frame at a time, so they have built-in buffers that enable them to store data arriving either from the computer or from the network until a frame is complete and ready for processing.

      Serial/parallel conversion
      The communication between the computer and the network interface adapter runs in parallel, that is, either 16 or 32 bits at a time, depending on the bus the adapter uses. Network communications, however, are serial (running one bit at a time), so the network interface adapter is responsible for performing the conversion between the two types of transmissions.

      Media access control
      The network interface adapter also implements the MAC mechanism that the data-link layer protocol uses to regulate access to the network medium. The nature of the MAC mechanism depends on the protocol used.

      Network protocols

      A networked computer must also have one or more protocol drivers (sometimes called a transport protocol or just a protocol). The protocol driver works between the upper-level network software and the network adapter to package data to be sent on the network.

      In most cases, for two computers to communicate on a network, they must use identical protocols. Sometimes, a computer is configured to use multiple protocols. In this case, two computers need only one protocol in common to communicate. For example, a computer running File and Printer Sharing for Microsoft Networks that uses both NetBEUI and TCP/IP can communicate with computers using only NetBEUI or TCP/IP.

      ISDN (Integrated Services Digital Network) adapters

      Integrated Services Digital Network adapters can be used to send voice, data, audio, or video over standard telephone cabling. ISDN adapters must be connected directly to a digital telephone network. ISDN adapters are not actually modems, since they neither modulate nor demodulate the digital ISDN signal.

      Like standard modems, ISDN adapters are available both as internal devices that connect directly to a computer's expansion bus and as external devices that connect to one of a computer's serial or parallel ports. ISDN can provide data throughput rates from 56 Kbps to 1.544 Mbps (using a T1 carrier service).

      

      ISDN hardware requires a NT (network termination) device, which converts network data signals into the signaling protocols used by ISDN. Some times, the NT interface is included, or integrated, with ISDN adapters and ISDN-compatible routers. In other cases, an NT device separate from the adapter or router must be implemented. ISDN works at the physical, data link, network, and transport layers of the OSI Model.

      WAPs (Wireless Access Point)

      A wireless network adapter card with a transceiver sometimes called an access point, broadcasts and receives signals to and from the surrounding computers and passes back and forth between the wireless computers and the cabled network.

      

      Access points act as wireless hubs to link multiple wireless NICs into a single subnet. Access points also have at least one fixed Ethernet port to allow the wireless network to be bridged to a traditional wired Ethernet network.

      Modems

      A modem is a device that makes it possible for computers to communicate over telephone lines. The word modem comes from Modulate and Demodulate. Because standard telephone lines use analog signals, and computers digital signals, a sending modem must modulate its digital signals into analog signals. The computers modem on the receiving end must then demodulate the analog signals into digital signals.

      

      Modems can be external, connected to the computers serial port by an RS-232 cable or internal in one of the computers expansion slots. Modems connect to the phone line using standard telephone RJ-11 connectors.

      Transceivers (media converters)

      Transceiver short for transmitter-receiver, a device that both transmits and receives analog or digital signals. The term is used most frequently to describe the component in local-area networks (LANs) that actually applies signals onto the network wire and detects signals passing through the wire. For many LANs, the transceiver is built into the network interface card (NIC). Some types of networks, however, require an external transceiver.

      

      In Ethernet networks, a transceiver is also called a Medium Access Unit (MAU). Media converters interconnect different cable types twisted pair, fiber, and Thin or thick coax, within an existing network. They are often used to connect newer 100-Mbps, Gigabit Ethernet, or ATM equipment to existing networks, which are generally 10BASE-T, 100BASE-T, or a mixture of both. They can also be used in pairs to insert a fiber segment into copper networks to increase cabling distances and enhance immunity to electromagnetic interference (EMI).

      Firewalls

      In computing, a firewall is a piece of hardware and/or software which functions in a networked environment to prevent some communications forbidden by the security policy, analogous to the function of firewalls in building construction.

      

      A firewall has the basic task of controlling traffic between different zones of trust. Typical zones of trust include the Internet (a zone with no trust) and an internal network (a zone with high trust). The ultimate goal is to provide controlled connectivity between zones of differing trust levels through the enforcement of a security policy and connectivity model based on the least privilege principle.

      There are three basic types of firewalls depending on:

      • whether the communication is being done between a single node and the network, or between two or more networks
      • whether the communication is intercepted at the network layer, or at the application layer
      • whether the communication state is being tracked at the firewall or not

      With regard to the scope of filtered communication these firewalls are exist:

      • Personal firewalls, a software application which normally filters traffic entering or leaving a single computer through the Internet.
      • Network firewalls, normally running on a dedicated network device or computer positioned on the boundary of two or more networks or DMZs (demilitarized zones). Such a firewall filters all traffic entering or leaving the connected networks.

      In reference to the layers where the traffic can be intercepted, three main categories of firewalls exist:

      • network layer firewalls An example would be iptables.
      • application layer firewalls An example would be TCP Wrapper.
      • application firewalls An example would be restricting ftp services through /etc/ftpaccess file

      These network-layer and application-layer types of firewall may overlap, even though the personal firewall does not serve a network; indeed, single systems have implemented both together.

      There's also the notion of application firewalls which are sometimes used during wide area network (WAN) networking on the world-wide web and govern the system software. An extended description would place them lower than application layer firewalls, indeed at the Operating System layer, and could alternately be called operating system firewalls.

      Lastly, depending on whether the firewalls track packet states, two additional categories of firewalls exist:

      • stateful firewalls
      • stateless firewalls

      Network layer firewalls

      Network layer firewalls operate at a (relatively low) level of the TCP/IP protocol stack as IP-packet filters, not allowing packets to pass through the firewall unless they match the rules. The firewall administrator may define the rules; or default built-in rules may apply (as in some inflexible firewall systems).

      A more permissive setup could allow any packet to pass the filter as long as it does not match one or more "negative-rules", or "deny rules". Today network firewalls are built into most computer operating system and network appliances.

      Modern firewalls can filter traffic based on many packet attributes like source IP address, source port, destination IP address or port, destination service like WWW or FTP. They can filter based on protocols, TTL values, netblock of originator, domain name of the source, and many other attributes.

      Application-layer firewalls

      Application-layer firewalls work on the application level of the TCP/IP stack (i.e., all browser traffic, or all telnet or ftp traffic), and may intercept all packets traveling to or from an application. They block other packets (usually dropping them without acknowledgement to the sender). In principle, application firewalls can prevent all unwanted outside traffic from reaching protected machines.

      By inspecting all packets for improper content, firewalls can even prevent the spread of the likes of viruses. In practice, however, this becomes so complex and so difficult to attempt (given the variety of applications and the diversity of content each may allow in its packet traffic) that comprehensive firewall design does not generally attempt this approach.

      Proxies

      A proxy device (running either on dedicated hardware or as software on a general-purpose machine) may act as a firewall by responding to input packets (connection requests, for example) in the manner of an application, whilst blocking other packets.

      

      Proxies make tampering with an internal system from the external network more difficult, and misuse of one internal system would not necessarily cause a security breach exploitable from outside the firewall (as long as the application proxy remains intact and properly configured). Conversely, intruders may hijack a publicly-reachable system and use it as a proxy for their own purposes; the proxy then masquerades as that system to other internal machines. While use of internal address spaces enhances security, crackers may still employ methods such as IP spoofing to attempt to pass packets to a target network.

      
      

      Network Operating Systems features UNIX Linux Netware

      A network operating system (NOS) provides services to clients over a network. Both the client/server and peer-to-peer networking models use network operating systems, and as such, NOSes must be able to handle typical network dutiessuch as the following:

      • Providing access to remote printers, managing which users are using which printers when, managing how print jobs are queued, and recognizing when devices aren't available to the network
      • Enabling and managing access to files on remote systems, and determining who can access what—and who can't
      • Granting access to remote applications and resources, such as the Internet, and making those resources seem like local resources to the user (the network is ideally transparent to the user)
      • Providing routing services, including support for major networking protocols, so that the operating system knows what data to send where
      • Monitoring the system and security, so as to provide proper security against viruses, hackers, and data corruption.
      • Providing basic network administration utilities (such as SNMP, or Simple Network Management Protocol), enabling an administrator to perform tasks involving managing network resources and users.

      UNIX / Linux

      UNIX, created originally by Bell Labs (under AT and T), is a powerful server operating system that can be used in peer-to-peer or client/server networks. UNIX was the first operating system written in the C programming language. Due to an antitrust ruling forbidding AT and T from releasing operating systems commercially, AT and T released UNIX upon its completion in 1974 to universities, mostly, enabling people to go in and actually view the source code to the system, which enabled coders to reconstruct the basic functions of the original UNIX operating system. From this practice, called reverse engineering, came Linux, which was first developed in the late 1980s by a young student at the University of Helsinki in Finland named Linus Torvalds.

      UNIX (and Linux by extension) systems offer the following features:

      • Fully protected multitasking: This means that UNIX can easily switch between tasks without the operating system crashing, because all UNIX processes are separate from those of the operating system. Even if an application crashes, unless it somehow manages to take down the X Windows system with it (which does happen), the operating system just keeps right on humming.
      • High performance and stability : Many servers running UNIX or Linux have run for years without crashing once. The multitasking capabilities of UNIX, along with the rapid rate at which the operating system matures (especially with Linux, which is free and can be changed by anyone), make UNIX or Linux a powerful solution, especially for server systems.
      • Multiuser capabilities : True multiuser systems enable different users to be logged in to the same system simultaneously. In UNIX and Linux, not only can a user log in to the same system at the same time as other users, that user can log in multiple times on the same system as the same user without the operating system batting an eyelash (such things are often necessary when administrating a network, particularly when managing users).
      • Tons of high-quality software : From Apache Server (a Web server that’s used on a whopping 6 in 10 major Web servers on the Internet) to the long-awaited Mozilla.org Mozilla 1.0 open source Web browser/e-mail software (Mozilla is an open source version of the venerated Netscape Communicator) to the powerful free Gimp graphics manipulation software, Linux is packed with tons of free, high-quality software. The trick is that, with UNIX/Linux, you give up compatibility with commercial software that’s available only for Windows and/or Macintosh, currently.
      • Easy customization : While other operating systems seem to offer less and less choice to the user about which applications to install with the operating system (Windows XP is this way), UNIX and especially Linux are the exact counterpoint to that model. With UNIX or Linux, you can actually customize your operating system kernel, stripping it down to just drivers and networking or installing everything possible.
      • Modular architecture : The modular architecture of UNIX (and especially Linux) is directly responsible for how customizable UNIX is. Modular really means just what it sounds like: The operating system is built with a kernel that attaches modules to itself based on what the user needs.
      • POSIX compliance: With a free operating system like UNIX, the different distributions (or flavors) of UNIX quickly became difficult to manage. Currently, hundreds of different implementations of UNIX are available. To enable programmers to have some idea of how to code their software such that it would run on any version of UNIX, the Institute of Electrical and Electronics Engineers, Inc. (IEEE) defined the Portable Operating System Interface (POSIX).
      • Use of TCP/IP as the standard protocol stack: UNIX overwhelmingly uses TCP/IP as the protocol stack of choice. If you consider that the vast majority of the servers that help make up the Internet are UNIX computers of one form or another, you start to get the idea why TCP/IP is so popular.
      • A shell interface: All versions of UNIX (at least those you care about for the exam) include a shell interface of some sort. If you have ever seen your computer use a completely black screen with white words written on it, that’s a shell interface. You simply type in commands at the prompt and hit Enter to execute those commands. The hard part in using these interfaces is simply the effort it takes to learn all of those rather cryptic commands. Making life even more difficult, UNIX is ultimately customizable and can use different shells. The bash shell (likely the most popular shell in use today) and the tcsh shell, for example, have different commands for the same action.
      • A graphical user interface: Although most versions of UNIX (such as Red Hat Linux) include a graphical user interface (GUI) these days, this has not always been the case. Historically, UNIX has been derided for its cryptic interface, and the advent of the GUI into popular UNIX systems was a direct result of this. Popular UNIX GUIs include KDE and GNOME. KDE is mostly used with Linux, but GNOME has versions for the Sun Solaris operating system, and therefore crosses the border from Linux into UNIX proper.
      • Support for dumb terminals: Traditionally, UNIX was used for dumb terminals, and just about all versions of UNIX still include this capability. The traditional dumb terminal model involves one central UNIX server that is used by remote terminals to execute applications. Basically, a user logs in to a UNIX system via Telnet or some other remote connectivity application and uses UNIX commands to tell the remote system what functions to perform. In this way, users can download and check e-mail via a text-based e-mail client such as Pine. The dumb terminal in this form isn’t used much anymore; Web browsers are definitely more than just dumb terminals— and Web browsers are now the clients most often seen by UNIX servers (at least those that make up the Internet). However, wireless devices such as cell phones and mobile Internet e-mail clients such as AOL’s Mobile Communicator device are good examples of modern dumb terminals. The devices have nearly no storage at all, and don’t carry large e-mail clients on the device; the message is simply transferred as text from one end to the other.

      Interoperability
      Open source software such as SAMBA is used to provide Windows users with Server Message Block (SMB) file sharing.

      Authentication:-Centralized login authentication

      File and Print Services
      Network File System (NFS) is a distributed file system that allows users to access files and directories located on remote computers and treat those files and directories as if they were local.
      LPR/LPD is the primary UNIX printing protocol used to submit jobs to the printer. The LPR component initiates commands such as "print waiting jobs," "receive job," and "send queue state," and the LPD component in the print server responds to them.

      Security
      With most Unix operating systems, the network services can be individually controlled to increase security.

      MAC OS X Server

      Client Support
      TCP/IP file sharing with Macintosh clients using Network File System (NFS), and File Transfer Apple File Protocol 3.0

      Interoperability
      Mac OS X Server uses the Open Source SAMBA to provide Windows users with Server Message Block (SMB) file sharing. Network File System (NFS) lets you make folders available to UNIX and Linux users.

      File and Print Services
      Mac OS X Server provides support for native Macintosh, Windows, UNIX, and Linux file sharing. Protocols supported include:

      • Apple file services (AFP 3.0) from any AppleShare client over TCP/IP
      • Windows (SMB/CIFS) file sharing using Samba
      • Network File System (NFS) for UNIX and Linux file access
      • Internet (FTP)

      Built-in print services can spool files to any PostScript-capable printer over TCP/IP, AppleTalk, or USB. Macintoshcustomers can use the LPR support in Print Center or the Desktop Printer utility to connect to a shared printer. Windows users can use their native SMB/CIFS protocol to connect to a shared printer.

      Print services for OS X Server

      Macintosh and UNIX (LPR/LPD)

      Windows (SMB/CIFS)

      Security

      • Multiple-user architecture and user-level access privileges.
      • Secure Sockets Layer (SSL) support provides encrypted and authenticated client/server communications.
      • Secure Shell (SSH) provides encryption and authentication for secure remote administration.
      • Kerberos support for centralized login authentication.

      Netware

      NetWare has been a great LAN operating system for years, but only recently (with NetWare 5.x has NetWare moved beyond the LAN to where it can easily be a part of larger networks. Until quite recently, Novell NetWare used to be the single most-used network operating system (NOS). However, first Windows NT, and Windows 2000 and Linux, have steadily eaten into the NetWare market share for network operating systems. Currently, all three operating systems have a roughly equal share of the network operating system market, which means that NetWare is still used in at least one-third of all server systems.
      NetWare features
      NetWare offers the following features :

      • Multiprocessor kernel: This feature enables one NetWare operating system to utilize multiple processors. This process is called symmetric multiprocessing (SMP). SMP enables processors to share memory and bus paths, even coordinating the processing of a single application in parallel.
      • NLMs: Where UNIX uses daemons and Windows uses services, NetWare uses NetWare Loadable Modules (or NLMs) to provide services from the server. NLMs are programs that run in the background on the server to provide consistent services to the network.
      • PCI Hot Plug: This feature enables administrators to dynamically configure PCI network components while the system is running. You can replace, upgrade, or add new cards with the Hot replace, Hot upgrade, and Hot expansion features, respectively.

      Client Support
      NetWare 5 comes with Novell Client software for three client platforms: DOS and Windows 3.1x, Windows 95/98, and Windows NT.

      Interoperability
      You can set the Novell Clients for Windows 95/98 and Windows NT to work with one of three network protocol options: IP only, IP and IPX, or IPX only.

      Authentication
      Centralized login authentication

      File and Print Services
      File Services NetWare offers two choices of mutually compatible file services: Novell Storage Services (NSS) and the traditional NetWare File System. Both kinds of file services let you store, organize, manage, access, and retrieve data on the network. NSS gathers all unpartitioned free space that exists on all the hard drives connected to your server, together with any unused space in NetWare volumes, and places it into a storage pool. You create NSS volumes from this storage pool during server installation or later through NWCONFIG. Novell Distributed Print Services (NDPS) is the default and preferred print system in NetWare. NDPS supports IP-based as well as IPX-based printing.

      Security
      Novell has support for a public key infrastructure built into NetWare 5 using a public certificate, developed by RSA Security.

      Windows

      Directory Services
      A directory service is a database of user accounts and other information that network administrators use to control access to shared network resources. When users connect to a network, they have to be authenticated before they can access network resources. Authentication is the process of checking the user's credentials (usually a user name and a password) against the directory. Users that supply the proper credentials are permitted access according to the permissions specified by the network administrator.

      Client Support
      Windows 3.x, Windows 95, Windows 98, and Windows NT Workstation 4.0 Windows 2000 Professional, Xp Pro. Vista Ultimate, Vista Business.

      Interoperability
      Windows 2000,2003,2008 Server supports UNIX, Novell NetWare, Windows NT Server 4.0, and Macintosh.

      Authentication
      Successful user authentication in a Windows 2000,2003,2008 computing environment consists of separate processes: interactive logon, which confirms the user's identification to either a domain account or a local computer, and network authentication, which confirms the user's identification to any network service that the user attempts to access.

      Types of authentication
      Kerberos V5 is used with either a password or a smart card for interactive logon. It is also the default method of network authentication for services.The Kerberos V5 protocol verifies both the identity of the user and network services Secure Socket Layer/Transport Layer Security (SSL/TLS) authentication, is used when a user attempts to access a secure Web server.

      File and Print Services
      You can add and maintain printers in Windows server using the print administration wizard, and you can add file shares using Active Directory management tools. Windows server also offers Distributed File Services, which let you combine files on more than one server into a single share.

      Active Directory 
      After many years of anticipation, Microsoft introduced an enterprise directory service in the Windows 2000 Server product line, called Active Directory. It uses a hierarchical tree design comprised of container and leaf objects. The fundamental unit of organization in Active Directory directory service is the domain, but; you can group domains together into a tree, and even group multiple trees together into a forest. Domains that are in the same tree automatically have bidirectional trust relationships established between them, which eliminates the need for administrators to create them manually. The trust relationships are also transitive , meaning that if Domain A trusts Domain B and Domain B trusts Domain C, then Domain A trusts Domain C.

      Security
      User-level security protects shared network resources by requiring that a security provider authenticate a user’s request to access resources. The domain controller , grants access to the shared resource by verifying that the user name and password are the same as those on the user account list stored on the network security provider. Because the security provider maintains a network-wide list of user accounts and passwords, each client computer does not have to store a list of accounts. Share-level security protects shared network resources on the computer with individually assigned passwords. For example, you can assign a password to a folder or a locally attached printer. If other users want to access it, they need to type in the appropriate password. If you do not assign a password to a shared resource, every user with access to the network can access that resource.

      Appleshare IP (Internet Protocol)

      Client Support
      TCP/IP file sharing with Macintosh clients using Network File System (NFS), and File Transfer Apple File Protocol 3.0.

      Interoperability
      Windows Server Message Block (SMB) file sharing.

      File and Print Services
      File Services:

      • Apple Filing Protocol (AFP) over TCP/IP and AppleTalk
      • Server Message Block (SMB) over TCP/IP
      • File Transfer Protocol (FTP) over TCP/IP

      Print Services:

      • PAP (AppleTalk)
      • LPR/LPD

      Application Support

      • HTTP
      • Mail (SMTP, POP, IMAP and Authenticated Post Office Protocol APOP)
      • Mac CGI

      • Tracert traceroute Ping Arp Netstat Nbtstat NetBIOS Ipconfig winipcfg nslookup

        Tracert / traceroute

        Tracert: Determines the path taken to a destination by sending Internet Control Message Protocol (ICMP) Echo Request messages to the destination with incrementally increasing Time to Live (TTL) field values. The path displayed is the list of near-side router interfaces of the routers in the path between a source host and a destination. The near-side interface is the interface of the router that is closest to the sending host in the path. Used without parameters, tracert displays help.
        This diagnostic tool determines the path taken to a destination by sending ICMP Echo Request messages with varying Time to Live (TTL) values to the destination. Each router along the path is required to decrement the TTL in an IP packet by at least 1 before forwarding it.

        Effectively, the TTL is a maximum link counter. When the TTL on a packet reaches 0, the router is expected to return an ICMP Time Exceeded message to the source computer. Tracert determines the path by sending the first Echo Request message with a TTL of 1 and incrementing the TTL by 1 on each subsequent transmission until the target responds or the maximum number of hops is reached. The maximum number of hops is 30 by default and can be specified using the -hparameter.

        The path is determined by examining the ICMP Time Exceeded messages returned by intermediate routers and the Echo Reply message returned by the destination. However, some routers do not return Time Exceeded messages for packets with expired TTL values and are invisible to the tracert command. In this case, a row of asterisks (*) is displayed for that hop.

        Examples:

        To trace the path to the host named www.google.co.in type:
        tracert www.google.co.in

        

        To trace the path to the host named www.google.com and prevent the resolution of each IP address to its name, type:
        tracert -d www.google.com

        To trace the path to the host named www.google.com and use the loose source route 10.12.0.1-10.29.3.1-10.1.44.1, type:
        tracert -j 10.12.0.1 10.29.3.1 10.1.44.1 www.google.com

        Syntax

        tracert [-d] [-h MaximumHops] [-j HostList] [-w Timeout] [TargetName]

        Parameters

        -d Prevents tracert from attempting to resolve the IP addresses of intermediate routers to their names. This can speed up the display of tracert results.

        -h MaximumHops Specifies the maximum number of hops in the path to search for the target (destination). The default is 30 hops.

        -j HostList Specifies that Echo Request messages use the Loose Source Route option in the IP header with the set of intermediate destinations specified in HostList. With loose source routing, successive intermediate destinations can be separated by one or multiple routers. The maximum number of addresses or names in the host list is 9. The HostList is a series of IP addresses (in dotted decimal notation) separated by spaces.

        -w Timeout Specifies the amount of time in milliseconds to wait for the ICMP Time Exceeded or Echo Reply message corresponding to a given Echo Request message to be received. If not received within the time-out, an asterisk (*) is displayed. The default time-out is 4000 (4 seconds).

        Ping

        Verifies IP-level connectivity to another TCP/IP computer by sending Internet Control Message Protocol (ICMP) Echo Request messages. The receipt of corresponding Echo Reply messages are displayed, along with round-trip times. Ping is the primary TCP/IP command used to troubleshoot connectivity, reachability, and name resolution.

        

        You can use ping to test both the computer name and the IP address of the computer. If pinging the IP address is successful, but pinging the computer name is not, you might have a name resolution problem. In this case, ensure that the computer name you are specifying can be resolved through the local Hosts file, by using Domain Name System (DNS) queries, or through NetBIOS name resolution techniques.
        To test a TCP/IP configuration by using the ping command:

        • To quickly obtain the TCP/IP configuration of a computer, open Command Prompt, and then type ipconfig . From the display of the ipconfig command, ensure that the network adapter for the TCP/IP configuration you are testing is not in a Media disconnected state.
        • At the command prompt, ping the loopback address by typing ping 127.0.0.1
        • Ping the IP address of the computer.
        • Ping the IP address of the default gateway. If the ping command fails, verify that the default gateway IP address is correct and that the gateway (router) is operational.
        • Ping the IP address of a remote host (a host that is on a different subnet). If the ping command fails, verify that the remote host IP address is correct, that the remote host is operational, and that all of the gateways (routers) between this computer and the remote host are operational.
        • Ping the IP address of the DNS server. If the ping command fails, verify that the DNS server IP address is correct, that the DNS server is operational, and that all of the gateways (routers) between this computer and the DNS server are operational.

        Arp

        Displays and modifies entries in the Address Resolution Protocol (ARP) cache, which contains one or more tables that are used to store IP addresses and their resolved Ethernet or Token Ring physical addresses. There is a separate table for each Ethernet or Token Ring network adapter installed on your computer.

        Syntax

        arp [-a [InetAddr] [-N IfaceAddr]] [-g [InetAddr] [-N IfaceAddr]] [-d InetAddr [IfaceAddr]] [-s InetAddr EtherAddr [IfaceAddr]]

        Parameters

        Used without parameters, ping displays help

        -a [InetAddr] [-N IfaceAddr] Displays current ARP cache tables for all interfaces. To display the ARP cache entry for a specific IP address, use arp -a with the InetAddr parameter, where InetAddr is an IP address. To display the ARP cache table for a specific interface, use the -N IfaceAddr parameter where IfaceAddr is the IP address assigned to the interface. The -N parameter is case-sensitive.

        -g [InetAddr] [-N IfaceAddr] Identical to -a.

        -d InetAddr [IfaceAddr] Deletes an entry with a specific IP address, where InetAddr is the IP address. To delete an entry in a table for a specific interface, use the IfaceAddr parameter where IfaceAddr is the IP address assigned to the interface. To delete all entries, use the asterisk (*) wildcard character in place of InetAddr.

        -s InetAddr EtherAddr [IfaceAddr] Adds a static entry to the ARP cache that resolves the IP address InetAddr to the physical address EtherAddr. To add a static ARP cache entry to the table for a specific interface, use the IfaceAddr parameter where IfaceAddr is an IP address assigned to the interface.

        Examples:

        To display the ARP cache tables for all interfaces, type:
        arp -a

        To display the ARP cache table for the interface that is assigned the IP address 10.0.0.99, type:

        

        arp -a -N 10.0.0.99

        To add a static ARP cache entry that resolves the IP address 10.0.0.80 to the physical address 00-AA-00-4F-2A-9C, type:
        arp -s 10.0.0.80 00-AA-00-4F-2A-9C

        

        Netstat

        Displays active TCP connections, ports on which the computer is listening, Ethernet statistics, the IP routing table, IPv4 statistics (for the IP, ICMP, TCP, and UDP protocols), and IPv6 statistics (for the IPv6, ICMPv6, TCP over IPv6, and UDP over IPv6 protocols).

         
        Netstat provides statistics for the following:

        • Proto - The name of the protocol (TCP or UDP).
        • Local Address - The IP address of the local computer and the port number being used. The name of the local computer that corresponds to the IP address and the name of the port is shown unless the -n parameter is specified. If the port is not yet established, the port number is shown as an asterisk (*).
        • Foreign Address - The IP address and port number of the remote computer to which the socket is connected. The names that corresponds to the IP address and the port are shown unless the -n parameter is specified. If the port is not yet established, the port number is shown as an asterisk (*).

        (state) Indicates the state of a TCP connection. The possible states are as follows:

        • CLOSE_WAIT
        • CLOSED
        • ESTABLISHED
        • FIN_WAIT_1
        • FIN_WAIT_2
        • LAST_ACK
        • LISTEN
        • SYN_RECEIVED
        • SYN_SEND
        • TIMED_WAIT

        Syntax

        netstat [-a] [-e] [-n] [-o] [-p Protocol] [-r] [-s] [Interval]

        Parameters

        Used without parameters, netstat displays active TCP connections.

        -a Displays all active TCP connections and the TCP and UDP ports on which the computer is listening.

        -e Displays Ethernet statistics, such as the number of bytes and packets sent and received. This parameter can be combined with -s.

        -n Displays active TCP connections, however, addresses and port numbers are expressed numerically and no attempt is made to determine names.

        -o Displays active TCP connections and includes the process ID (PID) for each connection. You can find the application based on the PID on the Processes tab in Windows Task Manager. This parameter can be combined with -a, -n, and -p.

        -p Shows connections for the protocol specified by Protocol. In this case, the Protocol can be tcp, udp, tcpv6, or udpv6. If this parameter is used with -s to display statistics by protocol, Protocol can be tcp, udp, icmp, ip, tcpv6, udpv6, icmpv6, or ipv6.

        -s Displays statistics by protocol. By default, statistics are shown for the TCP, UDP, ICMP, and IP protocols. If the IPv6 protocol for Windows XP is installed, statistics are shown for the TCP over IPv6, UDP over IPv6, ICMPv6, and IPv6 protocols. The -p parameter can be used to specify a set of protocols.

        -r Displays the contents of the IP routing table. This is equivalent to the route print command.

        Interval Redisplays the selected information every Interval seconds. Press CTRL+C to stop the redisplay. If this parameter is omitted, netstat prints the selected information only once.

        /? - Displays help at the command prompt.

        Nbtstat

        Displays NetBIOS over TCP/IP (NetBT) protocol statistics,
        

        NetBIOS

        NetBIOS name tables for both the local computer and remote computers, and the NetBIOS name cache. Nbtstat allows a refresh of the NetBIOS name cache and the names registered with Windows Internet Name Service (WINS).

        Nbtstat command-line parameters are case-sensitive.

        Syntax

        nbtstat [-a RemoteName] [-A IPAddress] [-c] [-n] [-r] [-R] [-RR] [-s] [-S] [Interval]

        Parameters

        Used without parameters, nbtstat displays help.

        -a RemoteName Displays the NetBIOS name table of a remote computer, where RemoteName is the NetBIOS computer name of the remote computer. The NetBIOS name table is the list of NetBIOS names that corresponds to NetBIOS applications running on that computer.

        -A IPAddress Displays the NetBIOS name table of a remote computer, specified by the IP address (in dotted decimal notation) of the remote computer.

        -c Displays the contents of the NetBIOS name cache, the table of NetBIOS names and their resolved IP addresses.

        -n Displays the NetBIOS name table of the local computer. The status of Registered indicates that the name is registered either by broadcast or with a WINS server.

        -r Displays NetBIOS name resolution statistics. On a Windows XP computer that is configured to use WINS, this parameter returns the number of names that have been resolved and registered using broadcast and WINS.

        -R Purges the contents of the NetBIOS name cache and then reloads the #PRE-tagged entries from the Lmhosts file.

        -RR Releases and then refreshes NetBIOS names for the local computer that is registered with WINS servers.

        -s Displays NetBIOS client and server sessions, attempting to convert the destination IP address to a name.

        -S Displays NetBIOS client and server sessions, listing the remote computers by destination IP address only.

        Interval Redisplays selected statistics, pausing the number of seconds specified in Interval between each display. Press CTRL+C to stop redisplaying statistics. If this parameter is omitted, nbtstat prints the current configuration information only once.

        /? - Displays help at the command prompt.

        Ipconfig

        Displays all current TCP/IP network configuration values and refreshes Dynamic Host Configuration Protocol (DHCP) and Domain Name System (DNS) settings. This command is most useful on computers that are configured to obtain an IP address automatically. This enables users to determine which TCP/IP configuration values have been configured by DHCP, Automatic Private IP Addressing (APIPA), or an alternate configuration.

        • If the Adapter name contains any spaces, use quotation marks around the adapter name (that is, "Adapter Name").
        • For adapter names, ipconfig supports the use of the asterisk (*) wildcard character to specify either adapters with names that begin with a specified string or adapters with names that contain a specified string.
        • For example, Local* matches all adapters that start with the string Local and *Con* matches all adapters that contain the string Con.

        Syntax

        ipconfig [/all] [/renew [Adapter]] [/release [Adapter]] [/flushdns] [/displaydns] [/registerdns] [/showclassid Adapter] [/setclassid Adapter [ClassID]]

        Parameters

        Used without parameters, ipconfig displays the IP address, subnet mask, and default gateway for all adapters.

        /all Displays the full TCP/IP configuration for all adapters. Without this parameter, ipconfig displays only the IP address, subnet mask, and default gateway values for each adapter. Adapters can represent physical interfaces, such as installed network adapters, or logical interfaces, such as dial-up connections.

        /renew [Adapter] Renews DHCP configuration for all adapters (if an adapter is not specified) or for a specific adapter if the Adapter parameter is included. This parameter is available only on computers with adapters that are configured to obtain an IP address automatically. To specify an adapter name, type the adapter name that appears when you use ipconfig without parameters.

        /release [Adapter] Sends a DHCPRELEASE message to the DHCP server to release the current DHCP configuration and discard the IP address configuration for either all adapters (if an adapter is not specified) or for a specific adapter if the Adapter parameter is included. This parameter disables TCP/IP for adapters configured to obtain an IP address automatically. To specify an adapter name, type the adapter name that appears when you use ipconfig without parameters.

        /flushdns Flushes and resets the contents of the DNS client resolver cache. During DNS troubleshooting, you can use this procedure to discard negative cache entries from the cache, as well as any other entries that have been added dynamically.

        /displaydns Displays the contents of the DNS client resolver cache, which includes both entries preloaded from the local Hosts file and any recently obtained resource records for name queries resolved by the computer. The DNS Client service uses this information to resolve frequently queried names quickly, before querying its configured DNS servers.

        /registerdns Initiates manual dynamic registration for the DNS names and IP addresses that are configured at a computer. You can use this parameter to troubleshoot a failed DNS name registration or resolve a dynamic update problem between a client and the DNS server without rebooting the client computer. The DNS settings in the advanced properties of the TCP/IP protocol determine which names are registered in DNS.

        /showclassid Adapter Displays the DHCP class ID for a specified adapter. To see the DHCP class ID for all adapters, use the asterisk (*) wildcard character in place of Adapter. This parameter is available only on computers with adapters that are configured to obtain an IP address automatically.

        /setclassid Adapter [ClassID] Configures the DHCP class ID for a specified adapter. To set the DHCP class ID for all adapters, use the asterisk (*) wildcard character in place of Adapter. This parameter is available only on computers with adapters that are configured to obtain an IP address automatically. If a DHCP class ID is not specified, the current class ID is removed.

        Examples:

        To display the basic TCP/IP configuration for all adapters, type:

        • ipconfig

        To display the full TCP/IP configuration for all adapters, type:

        • ipconfig /all

        To renew a DHCP-assigned IP address configuration for only the Local Area Connection adapter, type:

        • ipconfig /renew "Local Area Connection"

        To flush the DNS resolver cache when troubleshooting DNS name resolution problems, type:

        • ipconfig /flushdns

        To display the DHCP class ID for all adapters with names that start with Local, type:

        • ipconfig /showclassid Local

        To set the DHCP class ID for the Local Area Connection adapter to TEST, type:

        • ipconfig /setclassid "Local Area Connection" TEST

        winipcfg

        This utility allows users or adminstrators to see the current IP address and other useful information about your network configuration. You can reset one or more IP addresses. The Release or Renew buttons allow you to release or renew one IP address. If you want to release or renew all IP addresses click Release All or Renew All. When one of these buttons is clicked, a new IP address is obtained from either the DHCP service or from the computer assigning itself an automatic private IP address. To use the winipcfg utility:

        • Click Start,and then click Run and type winipcfg
        • Click More Info.
        • To see the addresses of the DNS servers the computer is configured to use, click the ellipsis (...) button to the right of DNS Servers.
        • To see address information for your network adapter(s), select an adapter from the list in Ethernet Adapter Information.

        nslookup

        Nslookup (Name Server lookup) is a UNIX shell command to query Internet domain name servers. 
        

        Definitions

        • Nameserver: These are the servers that the internet uses to find out more about the domain. Usually they are an ISP's computer.
        • Mailserver: Where email is sent to.
        • Webserver: The domains website.
        • FTPserver: FTP is file transfer protocol, this server is where files may be stored.
        • Hostname: The name of the host as given by the domain.
        • Real Hostname: This is hostname that you get by reverse resolving the IP address, may be different to the given hostname.
        • IP Address: Unique four numbered identifier that is obtained by resolving the hostname.

        
        

        Network Security implementation Port Blocking Filtering Authentication Encryption

        Port Blocking / Filtering

        A network layer firewall works as a packet filter by deciding what packets will pass the firewall according to rules defined by the administrator. Filtering rules can act on the basis of source and destination address and on ports, in addition to whatever higher-level network protocols the packet contains. Network layer firewalls tend to operate very fast, and transparently to users. Network layer firewalls generally fall into two sub-categories, stateful and stateless. Stateful firewalls hold some information on the state of connections (for example: established or not, initiation, handshaking, data or breaking down the connection) as part of their rules (e.g. only hosts inside the firewall can establish connections on a certain port).

        Stateless firewalls have packet-filtering capabilities but cannot make more complex decisions on what stage communications between hosts have reached. Stateless firewalls therefore offer less security. Stateless firewalls somewhat resemble a router in their ability to filter packets.

        Any normal computer running an operating system which supports packet filtering and routing can function as a network layer firewall. Appropriate operating systems for such a configuration include Linux, Solaris, BSDs or Windows Server.

        Authentication

        The process of identifying an individual, usually based on a username and password. In security systems, authentication is distinct from authorization , which is the process of giving individuals access to system objects based on their identity. Authentication merely ensures that the individual is who he or she claims to be, but says nothing about the access rights of the individual.

        Encryption

        Encryption is part of a larger process of encoding and decoding messages to keep information secure. This process, though commonly called encryption, is more correctly called cryptography, is the use of mathematical transformations to protect data. Cryptography is primarily a software-based solution and, in most cases, should not include significant hardware costs. It is a key tool in protecting privacy as it allows only authorized parties to view the data. Encryption is also used to ensure data integrity, as it protects data from being modified or corrupted.

        VLANs (Virtual Local Area Networks).

        A virtual LAN (VLAN) is a logical grouping of network devices in the same broadcast domain that can span multiple physical segments.A VLAN is a group of devices in the same broadcast domain or subnet. VLANs are good at logically separating traffic between different groups of users. VLANs contain/isolate broadcast traffic, where you need a router to move traffic between VLANs.

        Logically speaking, VLANs are subnets. A subnet, or a network, is a contained broadcast domain. A broadcast that occurs in one subnet will not be forwarded, by default, to another subnet.

        Routers, or layer 3 devices, provide this boundary function. Each of these subnets requires a unique network number. And to move from one network number to another, you need a router. In the case of broadcast domains and switches, each of these separate broadcast domains is a separate VLAN; therefore, you still need a routing function to move traffic between different VLANs.

        Extranets

        An extranet is a private network that uses Internet protocols, network connectivity, to securely share part of an organization's information or operations with suppliers, vendors, partners, customers or other businesses. An extranet can be viewed as part of a company's Intranet that is extended to users outside the company normally over the Internet. An extranet requires security and privacy. These can include firewalls, server management, the issuance and use of digital certificates or similar means of user authentication, encryption of messages, and the use of virtual private networks (VPNs) that tunnel through the public network. 
        Advantages

        • Extranets can improve organization productivity by automating processes that were previously done manually.
        • Extranets allow organization or project information to be viewed at times convenient for business partners, customers, employees, suppliers and other stake-holders.
        • Information on an extranet can be updated, edited and changed instantly. All authorised users therefore have immediate access to the most up-to-date information.

        Disadvantages

        • Extranets can be expensive to implement and maintain within an organisation
        • Security of extranets can be a big concern when dealing with valuable information.
        • Extranets can reduce personal contact (face-to-face meetings) with customers and business partners. This could cause a lack of connections made between people and a company

        • Intranet

          Intranets differ from "Extranets" in that the former is generally restricted to employees of the organization while extranets can generally be accessed by customers, suppliers, or other approved parties. An intranet is a private computer network that uses Internet protocols, network connectivity, to securely share part of an organization's information or operations with its employees. Sometimes the term refers only to the most visible service, the internal website. The same concepts and technologies of the Internet such as clients and servers running on the Internet protocol suite are used to build an intranet. HTTP and other Internet protocols are commonly used as well, especially FTP and e-mail.

          Antivirus Software.

          Antivirus software consists of computer programs that attempt to identify, thwart and eliminate computer viruses and other malicious software. Antivirus software typically uses two different techniques to accomplish this:
          • Examining files to look for known viruses matching definitions in a virus dictionary
          • Identifying suspicious behavior from any computer program which might indicate infection. Such analysis may include data captures, port monitoring and other methods.
          Most commercial antivirus software uses both of these approaches, with an emphasis on the virus dictionary approach.
          Dictionary Approach: 
          When the antivirus software looks at a file, it refers to a dictionary of known viruses that the authors of the antivirus software have identified. If a piece of code in the file matches any virus identified in the dictionary, then the antivirus software can take one of the following actions:
          • attempt to repair the file by removing the virus itself from the file
          • quarantine the file
          • delete the infected file.
          Suspicious Behavior Approach: 
          Unlike the dictionary approach, the suspicious behavior approach therefore provides protection against brand-new viruses that do not yet exist in any virus dictionaries. Most antivirus software are not using this approach much today. Using this approach the antivirus software:
          • Doesn't attempt to identify known viruses
          • Monitors the behavior of all programs.
          • If one program tries to write data to an executable program, the antivirus software can flag this suspicious behavior
          • alert a user and ask what to do.
          Analysis Approach:
        • Antivirus software could try to emulate the beginning of the code of each new executable that the system invokes before transferring control to that executable.
        • If the program seems to use self-modifying code or otherwise appears as a virus, one could assume that a virus has infected the executable. However, this method could result in a lot of false positives.

        • Fault Tolerance:

          Fault tolerance is the ability of a system to continue functioning when part of the system fails. Normally, fault tolerance is used in describing disk subsystems, but it can also apply to other parts of the system or the entire system. Fully fault-tolerant systems use redundant disk controllers and power supplies as well as fault-tolerant disk subsystems. You can also use an uninterruptible power supply (UPS) to safeguard against local power failure. Although the data is always available in a fault-tolerant system, you still need to make backups that are stored offsite to protect the data against disasters such as a fire.

          Network Redundancy

          Service interruptions on a network are not always the result of a computer or drive failure. Sometimes the network itself is to blame. For this reason, many larger internetworks are designed with redundant components that enable traffic to reach a given destination in more than one way. If a network cable is cut or broken, or if a router or switch fails, redundant equipment enables data to take another path to its destination. There are several ways to provide redundant paths. Typically, you have at least two routers or switches connected to each network, so that the computers can use either one as a gateway to the other segments. Example, you can build a network with two backbones. Each workstation can use either of the routers on its local segment as a gateway. You can also use this arrangement to balance the traffic on the two backbones by configuring half of the computers on each local area network (LAN) to use one of the routers as its default gateway and the other half to use the other router.

          Storage

          A redundant array of independent disks (RAID) is an example of a fault-tolerant storage device that uses data redundancy.

          RAID

          Redundant Array of Inexpensive (or Independent) Disks. A RAID array is a collection of drives which collectively act as a single storage system, which can tolerate the failure of a drive without losing data, and which can operate independently of each other.
          Level 0
          Referred to as striping, is not redundant. Data is split across drives, resulting in higher data throughput. Since no redundant information is stored, performance is very good, but the failure of any disk in the array results in all data loss.
          Level 1 
          Referred to as mirroring with 2 hard drives. It provides redundancy by duplicating all data from one drive on another drive. Performance is better than a single drive, but if either drive fails, no data is lost. This is a good entry-level redundant system, since only two drives are required.
          Level 2
          Which uses Hamming error correction codes, is intended for use with drives which do not have built-in error detection. All SCSI drives support built-in error detection, so this level is not needed if using SCSI drives.
          Level 3 
          Stripes data at a byte level across several drives, with parity stored on one drive. It is otherwise similar to level 4. Byte-level striping requires hardware support for efficient use.
          Level 4 
          Stripes data at a block level across several drives, with parity stored on one drive. The parity information allows recovery from the failure of any single drive. Performance is very good for reads. Writes, however, require that parity data be updated each time. This slows small random writes, in particular, though large writes or sequential writes are fairly fast.
          Level 5 
          Striping with distributed parity. Similar to level 4, but distributes parity among the drives. No single disk is devoted to parity. This can speed small writes in multiprocessing systems. Because parity data must be distributed on each drive during reads, the performance for reads tends to be considerably lower than a level 4 array.

          Disaster recovery

          Fault Tolerance

          Most people think about disaster recovery in terms of restoration of the damaged network, but it’s actually less expensive to prevent a disaster than to restore one.
          Fault tolerance is another term for redundancy. You can have redundant components within a server, redundant servers, and even redundant networks, in the case of a hot site. A fault-tolerant system simply has a spare part that takes over if another part fails. Fault tolerance can work for the following:

          Memory

          Some servers support error-correcting memory with a spare memory module to use in case of memory failure.

          Network interface cards (NICs).

          NICs can be redundant in two ways. They can share the network traffic, or one of the NICs can wait until the first fails before it kicks in.
          Redundant Array of Inexpensive Disks (RAID). 
          Data is mirrored, shared, or striped across multiple disks. Pay attention to these versions of RAID:
          RAID 1: 
          Mirroring disks connected to a single hard disk controller, or duplexing disks connected to two different hard disk controllers.
          RAID 5: 
          A group of three or more disks is combined into a volume with the disk striped across the disks, and parity is used to ensure that if any one of the disks fails, the remaining disks will still have all data available.
          Power supplies. 
          One power supply takes over if the original fails.
          Clusters. 
          Two or more servers are grouped to provide services as if the group were a single server. A cluster is transparent to end users. Usually, a server member of a cluster can take over for a failed partner with no impact on the network.

          Backup / restore

          Offsite storage
          A remote backup service, online backup service or managed backup service is a service that provides users with an online system for backing up and storing computer files. Managed backup providers are companies that have the software and server space for storing files.
          Hot and cold spares
          • A hot spare disk is running, ready to start working in the case of a failure.
          • A cold spare disk is not running.
          A hot spare is used as a failover mechanism to provide reliability in system configurations. The hot spare is active and connected as part of a working system. When a key component fails, the hot spare is switched into operation. Examples of hot spares are components such as networked printers, and hard disks. The equipment is powered on, or considered "hot", but not actively functioning in the system. In the case of a disk drive, data is being mirrored so when the hot spare takes over, the system continues to operate with minimal or no downtime.
          Hot Spare Disk 
          is a disk or group of disks used to automatically or manually, replace a failing or failed disk in a RAID configuration. The hot spare disk reduces the mean time to recovery (MTTR) for the RAID redundancy group, thus reducing the probability of a second disk failure and the resultant data loss that would occur in any singly redundant RAID (e.g., RAID-1, RAID-5, RAID-10).

          Hot, warm and cold sites

          A backup site is a location where a business can easily relocate following a disaster, such as fire, flood. There are three types of backup sites, including cold sites, warm sites, and hot sites. The differences between the types are determined by the costs and effort required to implement each.
          Hot Site is a duplicate of the original site of the business, with full computer systems as well as near-complete backups of user data. Following a disaster, the hot site exists so that the business can relocate with minimal losses to normal operations. Ideally, a hot site will be up and running within a matter of hours. This type of backup site is the most expensive to operate.
          Warm Site is a location where the business can relocate to after the disaster that is already stocked with computer hardware similar to that of the original site, but does not contain backed up copies of data and information.
          Cold Site is the most inexpensive type of backup site for a business to operate. It does not include backed up copies of data and information from the its original location, nor does it include hardware already set up. The lack of hardware contributes to the minimal startup costs of the cold site, but requires additional time following the disaster to have the operation running at a capacity close to that prior to the disaster.

          Full Form of hardware and networking devices

          In Comptia N+ or A+ exam you may have a question asking for full form. Beside exam in networking world we have to face several situation where need the extended or complete form of abbreviated form. Here we tried to list few most of them with their full form.
          
          

          ACPI	advanced configuration and power interface
          ACT	activity
          ADSL	asymmetrical digital subscriber line
          AGP	accelerated graphics port
          AMD	advanced micro devices
          AMR	audio modem riser
          APIPA	automatic private internet protocol addressing
          APM	advanced power management
          ARP	address resolution protocol
          ASR	automated system recovery
          AT	advanced technology
          ATA	advanced technology attachment
          ATAPI	advanced technology attachment packet interface
          ATM	asynchronous transfer mode
          ATX	advanced technology extended
          BIOS	basic input/output system
          BNC	Bayonet-Neill-Concelman or British Navel Connector
          BRI	basic rate interface
          BTX	balanced technology extended
          CCD	charged coupled device
          CD	compact disc
          CD-ROM	compact disc-read-only memory
          CD-RW	compact disc-rewritable
          CDFS	compact disc file system
          CMOS	complementary metal-oxide semiconductor
          CNR	communication network riser
          COM1	communication port 1
          CPU	central processing unit
          CRIMM	continuity-rambus inline memory module
          CRT	cathode-ray tube
          DAC	discretionary access control
          DB-25	serial communications D-shell connector, 25 pins
          DB-9	9 pin D shell connector
          DC	direct current
          DDOS	distributed denial of service
          DDR	double data-rate
          DDR RAM	double data-rate random access memory
          DDR SDRAM	double data-rate synchronous dynamic random access memory
          DFS	distributed file system
          DHCP	dynamic host configuration protocol
          DIMM	dual inline memory module
          DIN	Deutsche Industrie Norm
          DIP	dual inline package
          DLT	digital linear tape
          DLP	digital light processing
          DMA	direct memory access
          DNS	domain name service or domain name server
          DOS	disk operating system or denial of service
          DPMS	display power management signaling
          DRAM	dynamic random access memory
          DSL	digital subscriber line
          DVD	digital video disc or digital versatile disc
          DVD-RAM	digital video disc-random access memory
          DVD-ROM	digital video disc-read only memory
          DVD-R	digital video disc-recordable
          DVD-RW	digital video disc-rewritable
          DVI	digital visual interface
          ECC	error correction code
          ECP	extended capabilities port
          EEPROM	electrically erasable programmable read-only memory
          EFS	encrypting file system
          EIDE	enhanced integrated drive electronics
          EISA	extended industry standard architecture
          EMI	electromagnetic interference
          EMP	electromagnetic pulse
          EPROM	erasable programmable read-only memory
          EPP	enhanced parallel port
          ERD	emergency repair disk
          ESD	electrostatic discharge
          ESDI	enhanced small device interface
          EVGA	extended video graphics adapter/array
          EVDO	evolution data optimized or evolution data only
          FAT	file allocation table
          FAT12	12-bit file allocation table
          FAT16	16-bit file allocation table
          FAT32	32-bit file allocation table
          FDD	floppy disk drive
          FERPA	Family Educational Rights and Privacy Act
          Fn	Function (referring to the function key on a laptop)
          FPM	fast page-mode
          FRU	field replaceable unit
          FT	P file transfer protocol
          FQDN	fully qualified domain name
          GB	gigabyte
          GDI	graphics device interface
          GHz	gigahertz
          GUI	graphical user interface
          GPRS	general packet radio system
          GSM	global system for mobile communications
          HAL	hardware abstraction layer
          HCL	hardware compatibility list
          HDD	hard disk drive
          HDMi	high definition media interface
          HPFS	high performance file system
          HTML	hypertext markup language
          HTTP	hypertext transfer protocol
          HTTPS	hypertext transfer protocol over secure sockets layer
          I/O	input/output
          ICMP	internet control message protocol
          ICS	internet connection sharing
          ICR	intelligent character recognition
          IDE	integrated drive electronics
          IEEE	Institute of Electrical and Electronics Engineers
          IIS	Internet Information Services
          IMAP	internet mail access protocol
          IP	internet protocol
          IPCONFIG	internet protocol configuration
          IPP	internet printing protocol
          IPSEC	internet protocol security
          IPX	internetwork packet exchange
          IPX/SPX	internetwork packet exchange/sequenced packet exchange
          IR	infrared
          IrDA	Infrared Data Association
          IRQ	interrupt request
          ISA	industry standard architecture
          ISDN	integrated services digital network
          ISO	Industry Standards Organization
          ISP	internet service provider
          KB	kilobyte
          LAN	local area network
          LBA	logical block addressing
          LC	Lucent connector
          LCD	liquid crystal display
          LDAP	lightweight directory access protocol
          LED	light emitting diode
          LIP	or LiPoly lithium-ion polymer
          Li-on	lithium-ion
          LPD/LPR	line printer daemon / line printer remote
          LPT	line printer terminal
          LPT1	line printer terminal 1
          LPX	low profile extended
          LVD	low voltage differential
          MAC	media access control
          MAN	metropolitan area network
          MAPI	messaging application programming interface
          Mb	megabit
          MB	megabyte
          MBR	master boot record
          MBSA	Microsoft Baseline Security Analyzer
          MCR	multivariant curve resolution
          MFD	multi-function device
          MFP	multi-function product
          MHz	megahertz
          MicroDIMM	micro dual inline memory module
          MIDI	musical instrument digital interface
          MIME	multipurpose internet mail extension
          MLI	multiple link interface
          MMC	Microsoft management console
          MMX	multimedia extensions
          MP3	Moving Picture Experts Group Layer 3 Audio
          MPEG	Moving Picture Experts Group
          MSCONFIG	Microsoft configuration
          MSDS	material safety data sheet
          MUI	multilingual user interface
          NAS	network-attached storage
          NAT	network address translation
          NetBIOS	networked basic input/output system
          NetBEUI	networked basic input/output system extended user interface
          NFS	network file system
          NIC	network interface card
          NiCd	nickel cadmium
          NiMH	nickel metal hydride
          NLI	not logged in or natural language interface
          NLX	new low-profile extended
          NNTP	network news transfer protocol
          NTFS	new technology file system
          NTLDR	new technology loader
          NWLINK	Netware Link
          OCR	optical character recognition
          OEM	original equipment manufacturer
          OMR	optical mark recognition
          OS	operating system
          OSR	original equipment manufacturer service release
          PAN	personal area network
          PATA	parallel advanced technology attachment
          PC	personal computer
          PCI	peripheral component interconnect
          PCIe	peripheral component interconnect express
          PCIX	peripheral component interconnect extended
          PCL	printer control language
          PCMCIA	Personal Computer Memory Card International Association
          PDA	personal digital assistant
          PGA	pin grid array
          PGA2	pin grid array 2
          PIN	personal identification number
          PKI	public key infrastructure
          PnP	plug and play
          POP	post office protocol
          POP3	post office protocol 3
          POST	power-on self test
          POTS	plain old telephone service
          PPP	point-to-point protocol
          PPTP	point-to-point tunneling protocol
          PRI	primary rate interface
          PROM	programmable read-only memory
          PS/2	Personal System/2 connector
          PSTN	public switched telephone network
          PVC	permanent virtual circuit
          PXE	preboot execution environment
          QoS	quality of service
          RAID	redundant array of independent (or inexpensive) discs
          RAM	random access memory
          RAS	remote access service
          RBAC	role-based access control or rule-based access control
          RDRAM	RAMBUS dynamic random access memory
          RF	radio frequency
          RFI	radio frequency interference
          RGB	red green blue
          RIMM	RAMBUS inline memory module
          RIP	routing information protocol
          RIS	remote installation service
          RISC	reduced instruction set computer
          RJ	registered jack
          RJ-11	registered jack function 11
          RJ-45	registered jack function 45
          RMA	returned materials authorization
          ROM	read only memory
          RS-232 or RS-232C	recommended standard 232
          RTC	real-time clock
          SAN	storage area network
          SATA	serial advanced technology attachment
          SC	subscription channel
          SCSI	small computer system interface
          SCSI ID	small computer system interface identifier
          SD	card secure digital card
          SDRAM	synchronous dynamic random access memory
          SEC	single edge connector
          SFC	system file checker
          SGRAM	synchronous graphics random access memory
          SIMM	single inline memory module
          SLI	scalable link interface or system level integration or scanline interleave mode
          SMB	server message block or small to midsize business
          SMTP	simple mail transport protocol
          SNMP	simple network management protocol
          SoDIMM	small outline dual inline memory module
          SOHO	small office/home office
          SP	service pack
          SP1	service pack 1
          SP2	service pack 2
          SPDIF	Sony-Philips digital interface format
          SPGA	staggered pin grid array
          SPX	sequenced package exchange
          SRAM	static random access memory
          SSH	secure shell
          SSID	service set identifier
          SSL	secure sockets layer
          ST	straight tip
          STP	shielded twisted pair
          SVGA	super video graphics array
          SXGA	super extended graphics array
          TB	terabyte
          TCP	transmission control protocol
          TCP/IP	transmission control protocol/internet protocol
          TDR	time domain reflectometer
          TFTP	trivial file transfer protocol
          UART	universal asynchronous receiver transmitter
          UDF	user defined functions or universal disk format or universal data format
          UDMA	ultra direct memory access
          UDP	user datagram protocol
          UL	Underwriter’s Laboratory
          UNC	universal naming convention
          UPS	uninterruptible power supply
          URL	uniform resource locator
          USB	universal serial bus
          USMT	user state migration tool
          UTP	unshielded twisted pair
          UXGA	ultra extended graphics array
          VESA	Video Electronics Standards Association
          VFAT	virtual file allocation table
          VGA	video graphics array
          VoIP	voice over internet protocol
          VPN	virtual private network
          VRAM	video random access memory
          WAN	wide area network
          WAP	wireless application protocol
          WEP	wired equivalent privacy
          WIFI	wireless fidelity
          WINS	windows internet name service
          WLAN	wireless local area network
          WPA	wireless protected access
          WUXGA	wide ultra extended graphics array
          XGA	extended graphics array
          ZIF	zero-insertion-force
          ZIP	zigzag inline package