Monday, January 19, 2009


A computer network is an interconnection of a group of computers. Networks may be classified by what is called the network layer at which they operate according to basic reference models considered as standards in the industry such as the four-layer Internet Protocol Suite model. While the seven-layer Open Systems Interconnection (OSI) reference model is better known in academia, the majority of networks use the Internet Protocol Suite (IP) as their network model.
By scale
Computer networks may be classified according to the scale: Personal area network (PAN), Local Area Network (LAN), Campus Area Network (CAN), Metropolitan area network (MAN), or Wide area network (WAN). As Ethernet increasingly is the standard interface to networks, these distinctions are more important to the network administrator than the end user. Network administrators may have to tune the network, based on delay that derives from distance, to achieve the desired Quality of Service (QoS). The primary difference in the networks is the size.
Controller Area Networks are a special niche, as in control of a vehicle's engine, a boat's electronics, or a set of factory robots.
By connection method
Computer networks may be classified according to the hardware technology that is used to connect the individual devices in the network such as Optical fiber, Ethernet, Wireless LAN, HomePNA, or Power line communication.
Ethernets use physical wiring to connect devices. Often, they employ the use of hubs, switches, bridges, and routers.
Wireless LAN technology is built to connect devices without wiring. These devices use a radio frequency to connect.
By functional relationship (Network Architectures)
Computer networks may be classified according to the functional relationships which exist between the elements of the network, for example Active Networking, Client-server and Peer-to-peer (workgroup) architectures.
By network topology
Main article: Network Topology
Computer networks may be classified according to the network topology upon which the network is based, such as Bus network, Star network, Ring network, Mesh network, Star-bus network, Tree or Hierarchical topology network, etc.
Network Topology signifies the way in which intelligent devices in the network see their logical relations to one another. The use of the term "logical" here is significant. That is, network topology is independent of the "physical" layout of the network. Even if networked computers are physically placed in a linear arrangement, if they are connected via a hub, the network has a Star topology, rather than a Bus Topology. In this regard the visual and operational characteristics of a network are distinct; the logical network topology is not necessarily the same as the physical layout.
By protocol
Computer networks may be classified according to the communications protocol that is being used on the network. See the articles on List of network protocol stacks and List of network protocols for more information. For a development of the foundations of protocol design see Srikant 2004 [1] and Meyn 2007 [2]

Sunday, January 18, 2009

The Internet, Intranets, and Extranets

The Internet, Intranets, and Extranets
Internet : a collection of interconnected networks all freely exchanging information.

The ancestor of the internet was ARPANET, a project started by the U.S. Department of Defense (DOD) in 1969 as both an experiment in reliable networking and a means to link DOD and military research contractors, including a large number of universities doing military-funded research.
Internet Protocol (IP) :
Conventions that enable traffic to be routed one network to another as need.

How the Internet Works
The various networks that are linked to form the Internet work pretty much the same way, they pass data around in chunks called packets, each of which carries the address of its sender and its receiver. The set of conventions uses to pass packets from one host to another is known as the Internet Protocol (IP) , which operates at the network layer of the seven layer OSI model. Many protocols are connection with IP. The best known is the Transport Control Protocol (TCP), which operates at the transport layer and is used in combination with IP by most internet applications.
Backbone is one of the Internet’s high-speed , long distance communications links
Each computer on the Internet has an assigned address called its Uniform Resource Locator, or URL , to identify it from other hosts.
ตัวอย่างของ URL ก็เช่น
เรามาพิจารณาส่วนของ URL กันทีละส่วน
The “http” specifies the access method and tells your software to access this particular file using HyperText Transport Protocol.
The “www” part of address signifies that the address is associated with the World Wide Web.
The “” part of the address is the domain name that identifies the Internet host site and must adhere to strict rules. It always has at least two part separated by dots. For all countries except the USA , the rightmost part of the domain name is the country code (th for Thailand, au for Australia, etc.)

Accessing the Internet
There are three ways to connect to the Internet. ได้แก่
Connect via LAN server : This approach requires the user to install on his or her PC a network adapter card and Open Datalink Interface (ODI) or Network Driver Interface Specification (NDIS) packet drivers. These drivers allow multiple transport protocols to run on network card simultaneously. LAN servers are typically connected to the Internet at 56 Kbps or faster.
Connect via SLIP/PPP : This approach requires a modem an d the TCP/IP protocol software plus Serial Line Internet Protocol (SLIP) or Point to Point Protocol (PPP) software. SLIP and PPP are two communications protocols that transmit packets over telephone lines,allowing dial-up access to the Internet. The speed of this Internet connection is limited to the slower of your computer’s modem and the speed of your modem of the SLIP/PPP server to which you connect .
Connect via an on-line service : This approach requires nothing more than what is required to connect to any of the on-line information services- a modem, standard communications software, and on-line information service account.

Internet Service Providers
Internet Service Provider (ISP) is any company that provides individuals or organizations with access to the Internet.

E-Mail : E-Mail or Electronic mail enables you to send text, binary files, sound, and images to others.
Telnet : a terminal emulation protocol that enables users to log on to other computers on the Internet to gain access to public files.
File Transfer Protocol (FTP) : a protocol that describes a file transfer process between a host and a remote computer. FTP allows users to copy a file from one computer to another.
Usenet : a system closely allied with the Internet that use e-mail to provide a centralized news service. It is actually a protocol that describes how groups of messages can be stored on and sent between computers.
Newsgroups : an on-line discussion groups that focus on specific topic.
Chat room : a facility that enables two or more people to engage in interactive “conversations” over the Internet.
Voice-Over-IP (VOIP) : technology that enables network managers to route phone calls and fax transmissions over the same network they use for data.
Content Streaming : a method for transferring multimedia files over the Internet so that the data stream of voice and pictures plays continuously, without a break, or very few of them. It also enables users to browse large files in real time.

World Wide Web : a collection of tens of thousands of on dependently-owned computers that work together as one in an Internet service. These computers, called Web servers.
The Web is a menu-based system that uses the client/server model. It organizes Internet resources throughout the world into a series of menu pages, or screens, that appear on your computer.
The Web site is like a magazine, with a cover page called a homepage that has graphics, titles, and black and blue text.
Hypermedia : tools that connect the data on Web pages, allowing users to access topics in whatever order they wish.
Hypertext Markup Language (HTML) : is a standard page description language for Web pages.
HTML tags : codes that let the Web browser know how to format text: as a heading, as a list , or as body text and whether images, sound, and other elements should be inserted.
Web Browsers : software that creates a unique , hypermedia-based menu on your computer screen and provides a graphical interface to the Web.
Applet : a small program embedded in Web pages.
Search engine : a Web search tool.
Java : an object-oriented programming language from Sun Microsystems based on C++ that allows small programs –applet- to be embedded with in an HTML document.
Push technology : technology that enables users to automatically receive information over the Internet rather than searching for it using a browser.

Intranet : an internal corporate network built using Internet and World Wide Web standards and products that allows employees of an organization to gain access to corporate information.
Firewall : a device that sits between your internet network and the outside Internet and limits access into and out of your network based on your organization’s access policy.
Extranet : a network based on Web technologies that links selected resources of the intranet of a company with its customers, suppliers, or other business partners.

Summary of Internet, Intranet, and Extranet

Users Importance of reliability and performance Is there a need for user Authentication?
Internet Anyone Low No
Intranet Employees Low Yes
Extranet Selected business partners High Yes

Secured intranet and extranet access applications usually require the use of a virtual private network (VPN) . A virtual private network is a secure connection between two points across the Internet.
Tunneling : the process by which VPNs transfer information by encapsulating traffic in IP packets and sending the packets over the Internet.
Privacy and Security
Cryptography is the process of converting a message into a secret code and changing the encoded message back to regular text. The original conversion is called encryption. The unencoded message is called plaintext. The encoded message is call ciphertext.
Digital signature : an encryption technique used to meet the critical need for processing on-line financial transactions.


Network Topology
This sheet provides an introduction to network topologies. It covers each of the three major topologies (bus, star, and ring) in general terms. It does not go into detail on network architectures.
1. What is a topology?
2. What is a bus topology?
3. What is the difference between a regular bus and a local bus?
4. What is the difference between a regular bus and a local bus?
5. What are the key features of a star topology?
6. What are the advantages and disadvantages of a star topology?
7. What are the key features of a ring topology?
8. What are the advantages and disadvantages of a ring topology?
9. Why is a ring topology wired as a star?
10. What is a counterrotating ring?
11. Can you ‘mix’ topologies?
12. What are the costs considerations for choosing a topology?
1. What is a topology?
A topology refers to the manner in which the cable is run to individual workstations on the network. The dictionary defines topology as: the configurations formed by the connections between devices on a local area network (LAN) or between two or more LANs
There are three basic network topologies (not counting variations thereon): the bus, the star, and the ring.
It is important to make a distinction between a topology and an architecture. A topology is concerned with the physical arrangement of the network components. In contrast, an architecture addresses the components themselves and how a system is structured (cable access methods, lower level protocols, topology, etc.). An example of an architecture is 10baseT Ethernet which typically uses the start topology.
2. What is a bus topology?
A bus topology connects each computer (node) to a single segment trunk. A ‘trunk’ is a communication line, typically coax cable, which is referred to as the ‘bus.’ The signal travels from one end of the bus to the other. A terminator is required at each end to absorb the signal so it does not reflect back across the bus.

In a bus topology, signals are broadcast to all stations. Each computer checks the address on the signal (data frame) as it passes along the bus. If the signal’s address matches that of the computer, the computer processes the signal. If the address doesn’t match, the computer takes no action and the signal travels on down the bus.
Only one computer can ‘talk’ on a network at a time. A media access method called CSMA/CD is used to handle the collisions that occur when two signals are placed on the wire at the same time.
The bus topology is passive. In other words, the computers on the bus simply ‘listen’ for a signal; they are not responsible for moving the signal along.
A bus topology is normally implemented with coaxial cable.
3. What is the difference between a regular bus and a local bus?

In a regular bus, each computer is attached to the cable segment (called a backbone) by means of a drop cable (a shorter cable connecting the computer to the backbone) In a local bus, each computer is attached directly to the backbone in a daisy-chain configuration by means of a "T" connector. Peer-to-peer networks are often configured as a local bus.
4. What are the advantages and disadvantages of the bus topology?
Advantages of bus topology:
• Easy to implement and extend
• Well suited for temporary networks that must be set up in a hurry
• Typically the least cheapest topology to implement
• Failure of one station does not affect others
Disadvantages of bus topology:
• Difficult to administer/troubleshoot
• Limited cable length and number of stations
• A cable break can disable the entire network; no redundancy
• Maintenance costs may be higher in the long run
• Performance degrades as additional computers are added
5. What are the key features of a star topology?
All of the stations in a star topology are connected to a central unit called a hub.

The hub offers a common connection for all stations on the network. Each station has its own direct cable connection to the hub. In most cases, this means more cable is required than for a bus topology. However, this makes adding or moving computers a relatively easy task; simply plug them into a cable outlet on the wall.

If a cable is cut, it only affects the computer that was attached to it. This eliminates the single point of failure problem associated with the bus topology. (Unless, of course, the hub itself goes down.)
Star topologies are normally implemented using twisted pair cable, specifically unshielded twisted pair (UTP). The star topology is probably the most common form of network topology currently in use.
6. What are the advantages and disadvantages of a star topology?
Advantages of star topology:
• Easy to add new stations
• Easy to monitor and troubleshoot
• Can accommodate different wiring
Disadvantages of ring topology:
• Failure of hub cripples attached stations
• More cable required
7. What are the key features of a ring topology?
A ring topology consists of a set of stations connected serially by cable. In other words, it’s a circle or ring of computers. There are no terminated ends to the cable; the signal travels around the circle in a clockwise direction.

Note that while this topology functions logically as ring, it is physically wired as a star. The central connector is not called a hub but a Multistation Access Unit or MAU. (Don’t confuse a Token Ring MAU with a ‘Media Adapter Unit’ which is actually a transceiver.)
Under the ring concept, a signal is transferred sequentially via a "token" from one station to the next. When a station wants to transmit, it "grabs" the token, attaches data and an address to it, and then sends it around the ring. The token travels along the ring until it reaches the destination address. The receiving computer acknowledges receipt with a return message to the sender. The sender then releases the token for use by another computer.
Each station on the ring has equal access but only one station can talk at a time.
In contrast to the ‘passive’ topology of the bus, the ring employs an ‘active’ topology. Each station repeats or ’boosts’ the signal before passing it on to the next station.
Rings are normally implemented using twisted pair or fiber-optic cable.
8. What are the advantages and disadvantages of a ring topology?
Advantages of ring topology:
• Growth of system has minimal impact on performance
• All stations have equal access
Disadvantages of ring topology:
• Most expensive topology
• Failure of one computer may impact others
• Complex

9. Why is a ring topology wired as a star?
A ring topology has the same outward appearance as a star; all the stations are individually connected to a central location. In the star topology the device at the center is called a hub. In a ring topology, the center is called a MAU.
While they look the same, a closer examination reveals that the ring actually consists of a continuous circuit. Signals are passed along the circuit and accessed by stations in sequence. In a star topology the signal is split and sent out simultaneously to all stations.
The diagram below illustrates the continuous circuit of a ring.

10. What is a counterrotating ring?
A counterrotating ring is a ring topology that consists of two rings transmitting in opposite directions. The intent is to provide fault tolerance in the form of redundancy in the event of a cable failure. If one ring goes, the data can flow across to the other path, thereby preserving the ring.

11. Can you ‘mix’ topologies?
Yes, you can mix various topologies on the same network.
One very common example is a large Ethernet network with multiple hubs. Usually the hubs are located on different floors in a building or perhaps outside in another building. Each hub is wired in the typical star configuration. However, the hubs are connected together along a bus, typically referred to as a ‘backbone.’ The backbone between hubs might consist of fiber optic cable while the workstations are wired to each individual hub with UTP (unshielded twisted pair) cable.
12. What are the costs considerations for choosing a topology?
The following factors should be considered when choosing a topology:
• Installation
• Maintenance and troubleshooting
• Expected growth
• Distances
• Infrastructure
• Existing network
As a general rule, a bus topology is the cheapest to install, but may be more expensive to maintain because it does not provide for redundancy.
Copyright © 1997 & 1998 by David R. Frick & Company, CPA

Thursday, January 15, 2009

Network Interface Card (NIC)

A NIC (pronounced 'nick') is also known as a network card. It connects the computer to the cabling, which in turn links all of the computers on the network together. Each computer on a network must have a network card. Most modern network cards are 10/100 NICs and can operate at either 10Mbps or 100Mbps.Only NICs supporting a minimum of 100Mbps should be used in new installations schools. Computers with a wireless connection to a network also use a network card (see Advice Sheet 20 for more information on wireless networking).

Hub and Switch

A hub is a device used to connect a PC to the network. The function of a hub is to direct information around the network, facilitating communication between all connected devices. However in new installations switches should be used instead of hubs as they are more effective and provide better performance. A switch, which is often termed a 'smart hub'.

Switches and hubs are technologies or ‘boxes’ to which computers, printers, and other networking devices are connected. Switches are the more recent technology and the accepted way of building today's networks. With switching, each connection gets "dedicated bandwidth" and can operate at full speed. In contrast, a hub shares bandwidth across multiple connections such that activity from one PC or server can slow down the effective speed of other connections on the hub.
Now more affordable than ever, Dual-speed 10/100 autosensing switches are recommended for all school networks. Schools may want to consider upgrading any hub based networks with switches to improve network performance – ie speed of data on the network.

Wireless Networks

The term 'wireless network' refers to two or more computers communicating using standard network rules or protocols, but without the use of cabling to connect the computers together. Instead, the computers use wireless radio signals to send information from one to the other. A wireless local area network (WLAN) consists of two key components: an access point (also called a base station) and a wireless card. Information can be transmitted between these two components as long as they are fairly close together (up to 100 metres indoors or 350 metres outdoors). Suppliers would need to visit the schools and conduct a site survey. This will determine the number of base stations you need and the best place(s) to locate them. A site survey will also enable each supplier to provide you with a detailed quote. It is important to contact a number of different suppliers as prices, equipment and opinions may vary. When the term 'wireless network' is used today, it usually refers to a wireless local area network or WLAN. A WLAN can be installed as the sole network in a school or building. However, it can also be used to extend an existing wired network to areas where wiring would be too difficult or too expensive to implement, or to areas located away from the main network or main building. Wireless networks can be configured to provide the same network functionality as wired networks, ranging from simple peer-to-peer configurations to large-scale networks accommodating hundreds of users.

What are the advantages and disadvantages of a Wireless LAN?

Wireless LANs have advantages and disadvantages when compared with wired LANs. A wireless LAN will make it simple to add or move workstations, and to install access points to provide connectivity in areas where it is difficult to lay cable. Temporary or semi-permanent buildings that are in range of an access point can be wirelessly connected to a LAN to give these buildings connectivity. Where computer labs are used in schools, the computers (laptops) could be put on a mobile cart and wheeled from classroom to classroom, providing they are in range of access points. Wired network points would be needed for each of the access points.
A WLAN has some specific advantages:

• It is easier to add or move workstations
• It is easier to provide connectivity in areas where it is difficult to lay cable
• Installation can be fast and easy and can eliminate the need to pull cable through walls and ceilings
• Access to the network can be from anywhere in the school within range of an access point
• Portable or semi-permanent buildings can be connected using a wireless LAN
• Where laptops are used, the ‘computer suite’ can be moved from classroom to classroom on mobile carts
• While the initial investment required for wireless LAN hardware can be similar to the cost of wired LAN hardware, installation expenses can be significantly lower
• Where a school is located on more than one site (such as on two sides of a road), it is possible with directional antennae, to avoid digging trenches under roads to connect the sites
• In historic buildings where traditional cabling would compromise the façade, a wireless LAN can avoid drilling holes in walls
• Long-term cost benefits can be found in dynamic environments requiring frequent moves and changes
• They allows the possibility of individual pupil allocation of wireless devices that move around the school with the pupil.
WLANs also have some disadvantages:

• As the number of computers using the network increases, the data transfer rate to each computer will decrease accordingly
• As standards change, it may be necessary to replace wireless cards and/or access points
• Lower wireless bandwidth means some applications such as video streaming will be more effective on a wired LAN
• Security is more difficult to guarantee, and requires configuration
• Devices will only operate at a limited distance from an access point, with the distance determined by the standard used and buildings and other obstacles between the access point and the user
• A wired LAN is most likely to be required to provide a backbone to the wireless LAN; a wireless LAN should be a supplement to a wired LAN and not a complete solution
• Long-term cost benefits are harder to achieve in static environments that require few moves and changes
• It is easier to make a wired network ‘future proof’ for high data transfer.

Wireless Network Components

There are certain parallels between the equipment used to build a WLAN and that used in a traditional wired LAN. Both networks require network interface cards or network adapter cards. A wireless LAN PC card, which contains an in-built antenna, is used to connect notebook computers to a wireless network. Usually, this is inserted into the relevant slot in the side of the notebook, but some may be internal to the notebook. Desktop computers can also connect to a wireless network if a wireless network card is inserted into one of its internal PCI slots. In a wireless network, an 'access point' has a similar function to the hub in wired networks. It broadcasts and receives signals to and from the surrounding computers via their adapter card. It is also the point where a wireless network can be connected into an existing wired network.The most obvious difference between wireless and wired networks, however, is that the latter uses some form of cable to connect computers together. A wireless network does not need cable to form a physical connection between computers.
Wireless Network ConfigurationsWireless networks can be configured in an ad hoc/peer-to-peer arrangement or as a local area network.
Ad Hoc/Peer-to-Peer ConfigurationThis is the most basic wireless network configuration. It relies on the wireless network adapters installed in the computers that are communicating with each other. A computer within range of the transmitting computer can connect to it. However, if a number of computers are networked in this way, they must remain within range of each other. Even though this configuration has no real administration overhead, it should only be a consideration for very small installations.
Benefits and Educational UsesThe installation of cables is time consuming and expensive. The advantages of not doing so are apparent: the amount of work required and the time taken to complete it are significantly reduced the network is accessible in places where wiring would have been difficult or impossible with no cables linking computers together, cable-related faults and network downtime are minimisedWhere a wireless network is in place, teachers or students can have continuous access to the network, even as they move with their equipment from class to class. The space over which a wireless network operates is not planar but spherical. Therefore, in a multi-level site, network access is available in rooms above or below the access point, without the need for additional infrastructure. In a location within a school where network access is required occasionally, desktop computers fitted with wireless network cards can be placed on trolleys and moved from location to location. They can also be located in areas where group work is taking place. As they are connected to the network, documents and files can be shared, and access to the Internet is available, enhancing group project work. As the range of the wireless network extends outside the building, students and teachers can use wireless devices to gather and record data outside, e.g., as part of a science experiment or individual performance data as part of a PE class.
Technical and Purchasing ConsiderationsNetwork interface cards for wireless networks are more expensive than their wired counterparts. The cost of the access points has also to be considered. Wireless networks work at up top 54Mbps, whereas wired networks normally work at 100Mbps (Fast Ethernet). This data transmission rate is dependant on the number of users, the distance from the access point and the fabric of the building (metal structures in walls may have an impact). A wireless network will be noticeably slow when a group of users are transferring large files. This should be considered if multimedia applications are to be delivered over the network to a significant number of users. As the range of the network may extend beyond the walls of the building, it can be accessed from outside. Consideration should be given to what security features the equipment provides to ensure that only valid users have access to the network and that data is protected.

Wednesday, January 14, 2009


The purpose of the networking guidelines are as follows:

1. To assist schools in understanding the benefits of networking
2. To help schools place in context their current stage of networking development in their school. 3. To assist schools in planning the next stage of network development in their school.
4. To provide standard networking ‘models’ and best practice to schools that will assist schools in their network planning.

1.1. Basic of Networking
A computer network consists of a collection of computers, printers and other equipment that is connected together so that they can communicate with each other (see Advice Sheet 17 on the ICT Planning for schools pack). Fig 1 gives an example of a network in a school comprising of a local area network or LAN connecting computers with each other, the internet, and various servers.

Broadly speaking, there are two types of network configuration, peer-to-peer networks and client/server networks.
Peer to Peer Network
Peer-to-peer networks are more commonly implemented where less then ten computers are involved and where strict security is not necessary. All computers have the same status, hence the term 'peer', and they communicate with each other on an equal footing. Files, such as word processing or spreadsheet documents, can be shared across the network and all the computers on the network can share devices, such as printers or scanners, which are connected to any one computer.
Client/server networks are more suitable for larger networks. A central computer, or 'server', acts as the storage location for files and applications shared on the network. Usually the server is a higher than average performance computer. The server also controls the network access of the other computers which are referred to as the 'client' computers. Typically, teachers and students in a school will use the client computers for their work and only the network administrator (usually a designated staff member) will have access rights to the server.

Peer-to-Peer Networks vs Client/Server Networks
Peer-to-Peer Networks
· Easy to set up
· Less expensive to install
· Can be implemented on a wide range of operating systems
· More time consuming to maintain the software being used (as computers must be managed individually)
· Very low levels of security supported or none at all. These can be very cumbersome to set up, depending on the operating system being used
· Ideal for networks with less than 10 computers
· Does not require a server
· Demands a moderate level of skill to administer the network

Client/Server Networks
· More difficult to set up
· More expensive to install
· A variety of operating systems can be supported on the client computers, but the server needs to run an operating system that supports networking
· Less time consuming to maintain the software being used (as most of the maintenance is managed from the server)
· High levels of security are supported, all of which are controlled from the server. Such measures prevent the deletion of essential system files or the changing of settings
· No limit to the number of computers that can be supported by the network
· Requires a server running a server operating system
· Demands that the network administrator has a high level of IT skills with a good working knowledge of a server operating system

Components of a Network
A computer network comprises the following components:
A minimum of at least 2 computers
Cables that connect the computers to each other, although wireless communication is becoming more common (see Advice Sheet 20 for more information)
A network interface device on each computer (this is called a network interface card or NIC)
A ‘Switch’ used to switch the data from one point to another. Hubs are outdated and are little used for new installations.
Network operating system software

Structured Cabling
The two most popular types of structured network cabling are twisted-pair (also known as 10BaseT) and thin coax (also known as 10Base2). 10BaseT cabling looks like ordinary telephone wire, except that it has 8 wires inside instead of 4. Thin coax looks like the copper coaxial cabling that's often used to connect a Video Recorder to a TV.

10BaseT Cabling
When 10BaseT cabling is used, a strand of cabling is inserted between each computer and a hub. If you have 5 computers, you'll need 5 cables. Each cable cannot exceed 325 feet in length. Because the cables from all of the PCs converge at a common point, a 10BaseT network forms a star configuration.

Tuesday, January 13, 2009

Computer Network

1 Introduction to Computer Networks
2 Network Classification
2.2 Connection method
2.3 Functional relationship (Network Architectures)2.4 Network
3 Types of networks
3.1 Personal Area Network (PAN
3.2 Local Area Network (LAN
3.3 Campus Area Network (CAN)
3.5 Wide Area Network (WAN)/div>
3.6 Global Area Network (GAN
3.7 Virtual Private Network (VPN
3.8 Internetwork
3.8.1 Intranet
3.8.2 Extranet
3.8.3 Internet
4 Basic Hardware Components
4.1 Network Interface Cards
4.2 Repeaters
4.3 Hubs
4.4 Bridges
4.5 Switches
4.6 Routers

A computer network is a group of interconnected computers Networks may be classified according to a wide variety of characteristics. This article provides a general overview of some types and categories and also presents the basic components of a network.

Introduction to Computer Networks
A network is a collection of computers connected to each other. The network allows computers to communicate with each other and share resources and information. The Advance Research Projects Agency (ARPA) designed "Advanced Research Projects Agency Network" (ARPANET) for the United States Department of Defense. It was the first computer network in the world in late 1960's and early 1970's
Network Classification
The following list presents categories used for classifying networks.
caleBased on their scale, networks can be classified as Local Area Network (LAN), Wide Area Network (WAN), Metropolitan Area Network (MAN), Personal Area Network (PAN), Virtual Private Network (VPN), Campus Area Network (CAN), Storage Area Network (SAN), etc.

Connection method
Computer networks can also be classified according to the hardware and software technology that is used to interconnect the individual devices in the network, such as
Optical fiber, Ethernet, Wireless LAN, HomePNA, or Power line communication.
Ethernet uses physical wiring to connect devices. Frequently deployed devices include hubs, switches, bridges and/or routers.
Wireless LAN technology is designed to connect devices without wiring. These devices use
radio waves or infrared signals as a transmission medium.

Functional relationship (Network Architectures)
Computer networks may be classified according to the functional relationships which exist among the elements of the network, e.g.,
Active Networking, Client-server and Peer-to-peer (workgroup) architecture.

Network topology
Main article:
Network Topology
Computer networks may be classified according to the
network topology upon which the network is based, such as Bus network, Star network, Ring network, Mesh network, Star-bus network, Tree or Hierarchical topology network.
Network Topology signifies the way in which devices in the network see their logical relations to one another. The use of the term "logical" here is significant. That is, network topology is independent of the "physical" layout of the network. Even if networked computers are physically placed in a linear arrangement, if they are connected via a hub, the network has a Star topology, rather than a Bus Topology. In this regard the visual and operational characteristics of a network are distinct; the logical network topology is not necessarily the same as the physical layout. Networks may be classified based on the method of data used to convey the data, these include digital and analog networks.

Types of networks
Below is a list of the most common types of computer networks in order of scale.
Personal Area Network (PAN)
Main article:
Personal area network

A Personal Area Network (PAN) is a computer network used for communication among computer devices close to one person. Some examples of devices that are used in a PAN are printers, fax machines, telephones, PDAs and scanners. The reach of a PAN is typically about 20-30 feet (approximately 6-9 meters), but this is expected to increase with technology improvements.

Local Area Network (LAN)
Local Area Network (LAN) is a computer network covering a small physical area, like a home, office, or small group of buildings, such as a school, or an airport. Current LANs are most likely to be based on Ethernet technology. For example, a library may have a wired or wireless LAN for users to interconnect local devices (e.g., printers and servers) and to connect to the internet. On a wired LAN, PCs in the library are typically connected by category 5 (Cat5) cable, running the IEEE 802.3 protocol through a system of interconnected devices and eventually connect to the Internet. The cables to the servers are typically on Cat 5e enhanced cable, which will support IEEE 802.3 at 1 Gbit/s. A wireless LAN may exist using a different IEEE protocol, 802.11b, 802.11g or possibly 802.11n. The staff computers (bright green in the figure) can get to the color printer, checkout records, and the academic network and the Internet. All user computers can get to the Internet and the card catalog. Each workgroup can get to its local printer. Note that the printers are not accessible from outside their workgroup.
Typical library network, in a branching tree topology and controlled access to resources
All interconnected devices must understand the network layer (layer 3), because they are handling multiple subnets (the different colors). Those inside the library, which have only 10/100 Mbit/s Ethernet connections to the user device and a Gigabit Ethernet connection to the central router, could be called "layer 3 switches" because they only have Ethernet interfaces and must understand
IP. It would be more correct to call them access routers, where the router at the top is a distribution router that connects to the Internet and academic networks' customer access routers.
The defining characteristics of LANs, in contrast to WANs (wide area networks), include their higher data transfer rates, smaller geographic range, and lack of a need for leased telecommunication lines. Current Ethernet or other
IEEE 802.3 LAN technologies operate at speeds up to 10 Gbit/s. This is the data transfer rate. IEEE has projects investigating the standardization of 100 Gbit/s, and possibly 40 Gbit/s.

Campus Area Network (CAN)
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Campus Area Network
A Campus Area Network (CAN) is a computer network made up of an interconnection of local area networks (LANs) within a limited geographical area. It can be considered one form of a metropolitan area network, specific to an academic setting.
In the case of a university campus-based campus area network, the network is likely to link a variety of campus buildings including; academic departments, the university library and student residence halls. A campus area network is larger than a local area network but smaller than a wide area network (WAN), (in some cases).
The main aim of a campus area network is to facilitate students accessing internet and university resources. This is a network that connects two or more LANs but that is limited to a specific and contiguous geographical area such as a college campus, industrial complex, office building, or a military base. A CAN may be considered a type of MAN (metropolitan area network), but is generally limited to a smaller area than a typical MAN. This term is most often used to discuss the implementation of networks for a contiguous area. This should not be confused with a
Controller Area Network. A LAN connects network devices over a relatively short distance. A networked office building, school, or home usually contains a single LAN, though sometimes one building will contain a few small LANs (perhaps one per room), and occasionally a LAN will span a group of nearby buildings. In TCP/IP networking, a LAN is often but not always implemented as a single IP subnet.

Metropolitan Area Network (MAN)
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Metropolitan Area Network
A Metropolitan Area Network (MAN) is a network that connects two or more Local Area Networks or Campus Area Networks together but does not extend beyond the boundaries of the immediate town/city. Routers, switches and hubs are connected to create a Metropolitan Area Network.

Wide Area Network (WAN)
A Wide Area Network (WAN) is a computer network that covers a broad area (i.e., any network whose communications links cross metropolitan, regional, or national boundaries [1]). Less formally, a WAN is a network that uses routers and public communications links [1]. Contrast with personal area networks (PANs), local area networks (LANs), campus area networks (CANs), or metropolitan area networks (MANs) which are usually limited to a room, building, campus or specific metropolitan area (e.g., a city) respectively. The largest and most well-known example of a WAN is the Internet. A WAN is a data communications network that covers a relatively broad geographic area (i.e. one city to another and one country to another country) and that often uses transmission facilities provided by common carriers, such as telephone companies. WAN technologies generally function at the lower three layers of the OSI reference model: the physical layer, the data link layer, and the network layer.

Global Area Network (GAN)
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Global Area Network
A Global Area networks (GAN) specifications are in development by several groups, and there is no common definition. In general, however, a GAN is a model for supporting mobile communications across an arbitrary number of wireless LANs, satellite coverage areas, etc. The key challenge in mobile communications is "handing off" the user communications from one local coverage area to the next. In IEEE Project 802, this involves a succession of terrestrial Wireless local area networks (WLAN).[1]

Virtual Private Network (VPN)
A Virtual Private Network (VPN) is a computer network in which some of the links between nodes are carried by open connections or virtual circuits in some larger network (e.g., the Internet) instead of by physical wires. The link-layer protocols of the virtual network are said to be tunneled through the larger network when this is the case. One common application is secure communications through the public Internet, but a VPN need not have explicit security features, such as authentication or content encryption. VPNs, for example, can be used to separate the traffic of different user communities over an underlying network with strong security features.
A VPN may have best-effort performance, or may have a defined service level agreement (SLA) between the VPN customer and the VPN service provider. Generally, a VPN has a topology more complex than point-to-point.
A VPN allows computer users to appear to be editing from an IP address location other than the one which connects the actual computer to the Internet.

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ernetworking involves connecting two or more distinct computer networks or network segments via a common routing technology. The result is called an internetwork (often shortened to internet). Two or more networks or network segments connected using devices that operate at layer 3 (the 'network' layer) of the OSI Basic Reference Model, such as a router. Any interconnection among or between public, private, commercial, industrial, or governmental networks may also be defined as an internetwork.
In modern practice, the interconnected networks use the Internet Protocol. There are at least three variants of internetwork, depending on who administers and who participates in them:
Intranets and extranets may or may not have connections to the Internet. If connected to the Internet, the intranet or extranet is normally protected from being accessed from the Internet without proper authorization. The Internet is not considered to be a part of the intranet or extranet, although it may serve as a portal for access to portions of an extranet.

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An intranet is a set of networks, using the Internet Protocol and IP-based tools such as web browsers and file transfer applications, that is under the control of a single administrative entity. That administrative entity closes the intranet to all but specific, authorized users. Most commonly, an intranet is the internal network of an organization. A large intranet will typically have at least one web server to provide users with organizational information.

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An extranet is a network or internetwork that is limited in scope to a single organization or entity but which also has limited connections to the networks of one or more other usually, but not necessarily, trusted organizations or entities (e.g. a company's customers may be given access to some part of its intranet creating in this way an extranet, while at the same time the customers may not be considered 'trusted' from a security standpoint). Technically, an extranet may also be categorized as a CAN, MAN, WAN, or other type of network, although, by definition, an extranet cannot consist of a single LAN; it must have at least one connection with an external network.

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The Internet is a specific internetwork. It consists of a worldwide interconnection of governmental, academic, public, and private networks based upon the networking technologies of the Internet Protocol Suite. It is the successor of the Advanced Research Projects Agency Network (ARPANET) developed by DARPA of the U.S. Department of Defense. The Internet is also the communications backbone underlying the World Wide Web (WWW). The 'Internet' is most commonly spelled with a capital 'I' as a proper noun, for historical reasons and to distinguish it from other generic internetworks.
Participants in the Internet use a diverse array of methods of several hundred documented, and often standardized, protocols compatible with the
Internet Protocol Suite and an addressing system (IP Addresses) administered by the Internet Assigned Numbers Authority and address registries. Service providers and large enterprises exchange information about the reachability of their address spaces through the Border Gateway Protocol (BGP), forming a redundant worldwide mesh of transmission paths.

Basic Hardware Components
All networks are made up of basic hardware building blocks to interconnect network
Main article: Network card
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A repeater is an electronic device that receives a signal and retransmits it at a higher power level, or to the other side of an obstruction, so that the signal can cover longer distances without degradation. In most twisted pair ethernet configurations, repeaters are required for cable runs longer than 100 meters away from the computer.

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Network hub
A hub contains multiple ports. When a packet arrives at one port, it is copied to all the ports of the hub for transmission. When the packets are copied, the destination address in the frame does not change to a broadcast address. It does this in a rudimentary way: It simply copies the data to all of the Nodes connected to the hub.[2]

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Network bridge
A network bridge connects multiple
network segments at the data link layer (layer 2) of the OSI model. Bridges do not promiscuously copy traffic to all ports, as hubs do, but learn which MAC addresses are reachable through specific ports. Once the bridge associates a port and an address, it will send traffic for that address only to that port. Bridges do send broadcasts to all ports except the one on which the broadcast was received.
Bridges learn the association of ports and addresses by examining the source address of frames that it sees on various ports. Once a frame arrives through a port, its source address is stored and the bridge assumes that MAC address is associated with that port. The first time that a previously unknown destination address is seen, the bridge will forward the frame to all ports other than the one on which the frame arrived.
Bridges come in three basic types:
Local bridges: Directly connect local area networks (LANs)
Remote bridges: Can be used to create a wide area network (WAN) link between LANs. Remote bridges, where the connecting link is slower than the end networks, largely have been replaced by routers.
Wireless bridges: Can be used to join LANs or connect remote stations to LANs.

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Network switch
A switch is a device that performs switching. Specifically, it forwards and filters
OSI layer 2 datagrams (chunk of data communication) between ports (connected cables) based on the MAC addresses in the packets.[3] This is distinct from a hub in that it only forwards the datagrams to the ports involved in the communications rather than all ports connected. Strictly speaking, a switch is not capable of routing traffic based on IP address (layer 3) which is necessary for communicating between network segments or within a large or complex LAN. Some switches are capable of routing based on IP addresses but are still called switches as a marketing term. A switch normally has numerous ports, with the intention being that most or all of the network is connected directly to the switch, or another switch that is in turn connected to a switch.[4]
Switch is a marketing term that encompasses routers and bridges, as well as devices that may distribute traffic on load or by application content (e.g., a Web
URL identifier). Switches may operate at one or more OSI model layers, including physical, data link, network, or transport (i.e., end-to-end). A device that operates simultaneously at more than one of these layers is called a multilayer switch.
Overemphasizing the ill-defined term "switch" often leads to confusion when first trying to understand networking. Many experienced network designers and operators recommend starting with the logic of devices dealing with only one protocol level, not all of which are covered by OSI. Multilayer device selection is an advanced topic that may lead to selecting particular implementations, but multilayer switching is simply not a real-world design concept.

Routers Main article: Router
Routers are networking devices that forward data packets between networks using headers and forwarding tables to determine the best path to forward the packets. Routers work at the
network layer of the TCP/IP model or layer 3 of the OSI model. Routers also provide interconnectivity between like and unlike media (RFC 1812). This is accomplished by examining the Header of a data packet, and making a decision on the next hop to which it should be sent (RFC 1812) They use preconfigured static routes, status of their hardware interfaces, and routing protocols to select the best route between any two subnets. A router is connected to at least two networks, commonly two LANs or WANs or a LAN and its ISP's network. Some DSL and cable modems, for home (and even office) use, have been integrated with routers to allow multiple home/office computers to access the Internet through the same connection. Many of these new devices also consist of wireless access points (waps) or wireless routers to allow for IEEE 802.11g/b/n wireless enabled devices to connect to the network without the need for cabled connections.
types of the computer networking
There are two main types of the computer networking client-server and peer to peer. In the client server computing, a computer plays a major role known as server, where the files, data in the form of web pages, docs or spread sheet files, video, database & resources are placed.
All the other computers in the client/server networks are called clients and they get the data from the server. In the peer to peer networks all the computers play the same role and no computer act as a centralized server. In the major businesses around the world client-server networks model is in major use.
A network topology defines the structure, design or layout of a network. There are different topologies like bus, ring, star, mesh, hybrid etc. The star topology is most commonly used topology. In the star topology, all the computers in the network are connected with a centralized device such as hub or switch. Thus forms a star like structure. If the hubs/switch fails to work for any reason then all the connectivity and communication between the computers will be halted.
A common communication language is used by the computers and the communication devices is known as protocols. The most commonly used and popular protocol on the internet and in the home and other networks is called TCP/IP. TCP/IP is not a single protocol but it is a suite of several protocols.A computer network can be a wired or wireless and TCP/IP protocol can work both in types of a network.Data flow or communication can be divided into seven logical layers called OSI layers model that was developed by Intel and Xerox Corporation and was standardized by ISO.
1. Application layer 2. Presentation layer3. Session layer4. Transport layer 5. Network layer 6. Data Link layer a. Media access control sub-layer b. Logical link control sub-layer 7. Physical layerA network can be divided into different scales and ranges and it depends on the requirement of the organization and the geographical location. Computer Network can be divided into Local Area Network, Personal Area Network, Campus Area Network, Wireless Local Area Network, Metropolitan Area Network and Wide Area Network.There are several communication connection methods like HomePNA, Power line communication, Ethernet and Wifi connection method.A network can also be categorized into several different types based on the services it provides like Server farms, Storage area networks, Value control networks, Value-Added networks, SOHO networks, Wireless and Jungle networks.
Building a simple computer network

A simple computer network may be constructed from two computers by adding a network adapter (Network Interface Controller (NIC)) to each computer and then connecting them together with a special cable called a crossover cable. This type of network is useful for transferring information between two computers that are not normally connected to each other by a permanent network connection or for basic home networking applications. Alternatively, a network between two computers can be established without dedicated extra hardware by using a standard connection such as the RS-232 serial port on both computers, connecting them to each other via a special crosslinked null modem cable.
Practical networks generally consist of more than two interconnected computers and generally require special devices in addition to the Network Interface Controller that each computer needs to be equipped with. Examples of some of these special devices are hubs, switches and routers.
Ancillary equipment used by networks
To keep a network operating, to diagnose failures or degradation, and to circumvent problems, networks may have a wide-ranging amount of ancillary equipment.
Providing Electrical Power
Individual network components may have surge protectors - an appliance designed to protect electrical devices from voltage spikes. Surge protectors attempt to regulate the voltage supplied to an electric device by either blocking or shorting to ground voltage above a safe threshold.
Beyond the surge protector, network elements may have uninterruptible power supplies (UPS), which can be anywhere from a line-charged battery to take the element through a brief power dropout, to an extensive network of generators and large battery banks that can protect the network for hours or days of commercial power outages.
A network as simple as two computers linked with a crossover cable has several points at which the network could fail: either network interface, and the cable. Large networks, without careful design, can have many points at which a single failure could disable the network.
When networks are critical the general rule is that they should have no single point of failure. The broad factors that can bring down networks, according to the Software Engineering Institute at Carnegie-Mellon University:
Attacks: these include software attacks by various miscreants (e.g., malicious hackers, computer criminals) as well as physical destruction of facilities.
Failures: these are in no way deliberate, but range from human error in entering commands, bugs in network element executable code, failures of electronic components, and other things that involve deliberate human action or system design.
Accidents: Ranging from spilling coffee into a network element to a natural disaster or war that destroys a data center, these are largely unpredictable events. Survivability from severe accidents will require physically diverse, redundant facilities. Among the extreme protections against both accidents and attacks are airborne command posts and communications relays[7], which either are continuously in the air, or take off on warning. In like manner, systems of communications satellites may have standby spares in space, which can be activated and brought into the constellation.
Dealing with Power Failures
One obvious form of failure is the loss of electrical power. Depending on the criticality and budget of the network, protection from power failures can range from simple filters against excessive voltage spikes, to consumer-grade Uninterruptible Power Supplies(UPS) that can protect against loss of commercial power for a few minutes, to independent generators with large battery banks. Critical installations may switch from commercial to internal power in the event of a brownout,where the voltage level is below the normal minimum level specified for the system. Systems supplied with three-phase electric power also suffer brownouts if one or more phases are absent, at reduced voltage, or incorrectly phased. Such malfunctions are particularly damaging to electric motors. Some brownouts, called voltage reductions, are made intentionally to prevent a full power outage.
Some network elements operate in a manner to protect themselves and shut down gracefully in the event of a loss of power. These might include noncritical application and network management servers, but not true network elements such as routers. UPS may provide a signal called the "Power-Good" signal. Its purpose is to tell the computer all is well with the power supply and that the computer can continue to operate normally. If the Power-Good signal is not present, the computer shuts down. The Power-Good signal prevents the computer from attempting to operate on improper voltages and damaging itself
To help standardize approaches to power failures, the Advanced Configuration and Power Interface (ACPI) specification is an open industry standard first released in December 1996 developed by HP, Intel, Microsoft, Phoenix and Toshiba that defines common interfaces for hardware recognition, motherboard and device configuration and power management.
Monitoring and Diagnostic Equipment
Networks, depending on their criticality and the skill set available among the operators, may have a variety of temporarily or permanently connected performance measurement and diagnostic equipment. Routers and bridges intended more for the enterprise or ISP market than home use, for example, usually record the amount of traffic and errors experienced on their interfaces.