User:Mlewis000/Computer network draft

RJ-45 patchcord of the type commonly used to connect network devices.

For the article on computer networking see Computer networking.

A computer network is two or more computers connected together using a telecommunication system for the purpose of communicating and sharing resources.

It is interesting to note that there is some disagreement among experts in the field of computer networking concerning whether two computers that are connected together using some form of communications medium constitute a computer network and, therefore, some sources will state that three computers connected together are required to form a computer network. Some would also say that a computer networked to a non-computing device (e.g., a computer networked to a printer via an Ethernet link) is also a computer network, although this article does not currently address this configuration in the definition of a computer network. This article uses the definition which requires only two computers to be connected together to form a computer network since the same basic functions are generally present in this case as with larger numbers of connected computers.

Basic Computer Network Building Blocks

Computers

Many of the components of which an average computer network is composed are individual computers, which are generally either workstations or servers.

Types of Workstations

There are many types of workstations that may be incorporated into a particular network, some of which have high-end displays, multiple CPUs, large amounts of RAM, large amounts of hard drive storage space, or other enhancements required for special data processing tasks, graphics, or other resource intensive applications. See also Network computer.

Types of Servers

The following is a list of some of the more common types of servers and their purpose.

File Server

Stores various types of files and distributes them to other clients on the network.

Print server

Controls and manages one or more printers and accepts print jobs from other network clients, spooling the print jobs, and performing most or all of the other functions that a workstation would perform to accomplish a printing task if the printer were connected directly to the workstation's printer port.

Mail Server

Stores, sends, receives, routes, and performs other email related operations for other clients on the network.

Fax Server

Stores, sends, receives, routes, and performs other functions necessary for the proper transmission, reception, and distribution of faxes.

Telephony Server

Performs telephony related functions such as answering calls automatically, performing the functions of an interactive voice response system, storing and serving voice mail, routing calls between the Public Switched Telephone Network (PSTN) and the network or the Internet (e.g., voice-overIP (VoIP) gateway), etc.

Proxy Server

Performs some type of function on behalf of other clients on the network to increase the performance of certain operations (e.g., prefetching and caching documents or other data that is requested very frequently) or as a security precaution to isolate network clients from external threats.

Remote Access Server (RAS)

Monitors modem lines or other network communications channels for requests to connect to the network from a remote location, answers the incoming telephone call or acknowledges the network request, and performs the necessary security checks and other procedures necessary to log a user onto the network.

Application Server

Performs the data processing or business logic portion of a client application, accepting instructions for what operations to perform from a workstation and serving the results back to the workstation, while the workstation performs the user interface or GUI portion of the processing (i.e., the presentation logic) that is required for the application to work properly.

Web Server

Stores HTML documents, images, text files, scripts, and other Web related data (collectively known as content), and distributes this content to other clients on the network on request.

Backup Server

Has network backup software installed and has large amounts of hard drive storage or other forms of storage (tape, etc.) available to it to be used for the purpose of insuring that data loss does not occur in the network.

Printers

Many printers are capable of acting as part of a computer network without any other device, such as a print server, to act as an intermediary between the printer and the device that is requesting a print job to be completed.

Dumb Terminals

Many networks use dumb terminals instead of workstations either for data entry and display purposes or in some cases where the application runs entirely on the server.

Other Devices

There are many other types of devices that may be used to build a network, many of which require an understanding of more advanced computer networking concepts before they are able to be easily understood (e.g., hubs, routers, bridges, switches, hardware firewalls, etc.).

Building a Computer Network

A Simple 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 small home networking applications.

Practical Networks

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 listed above under Basic Computer Network Building Blocks / Other devices.

Types of Networks:

Below is a list of the more common types of computer networks, although there are other types that are not listed.

Local Area Network (LAN):

A network that is limited to a relatively small spatial area such as a room, a single building, a ship, or an aircraft.

Note: For administrative purposes, large LANs are generally divided into smaller logical segments called workgroups. A workgroup is a group of computers that share a common set of resources within a LAN.

Campus Area Network (CAN):

A network that connects two or more LANs but that is limited to a specific (possibly private) geographical area such as a college campus, industrial complex, or a military base.

Note: A CAN is generally limited to an area that is smaller than a Metropolitan Area Network (MAN).

Metropolitan Area Network (MAN):

A network that connects two or more LANs or CANs together but that does not extend beyond the boundaries of the immediate town, city, or metropolitan area.

Wide Area Network (WAN):

A network that covers a broad geographical area (i.e., any network whose communications links cross metropolitan, regional, or national boundaries) or, less formally, a network that uses routers and public communications links.

Types of WANs:

Centralized:

A centralized WAN consists of a central computer that is connected to dumb terminals and / or other types of terminal devices.

Distributed:

A distributed WAN consists of two or more computers in different locations and may also include connections to dumb terminals and other types of terminal devices.

Internetwork:

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 or layer 3 switch.

Note: Any interconnection among or between public, private, commercial, industrial, or governmental networks may also be defined as an internetwork.

Internet, The:

A specific internetwork, consisting of a worldwide interconnection of governmental, academic, public, and private networks based upon the Advanced Research Projects Agency Network (ARPANET) developed by ARPA of the U.S. Department of Defense – also known as the World Wide Web (WWW) or the 'Internet' with a capital 'I' to distinguish it from other generic internetworks.

Intranet:

A network or internetwork that is limited in scope to a single organization or entity or, also, a network or internetwork that is limited in scope to a single organization or entity and which uses the TCP/IP protocol suite, HTTP, FTP, and other network protocols and software commonly used on the Internet.

Note: Intranets may also be categorized as a LAN, CAN, MAN, WAN, or other type of network.

Extranet:

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 provided access to some part of its intranet thusly creating an extranet while at the same time the customers may not be considered 'trusted' from a security standpoint).

Note: 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, since an extranet must have at least one connection with an outside network.

It is interesting to note that Intranets and extranets may or may not have connections to the Internet, but if they are connected to the Internet, the intranet or extranet is normally protected from being accessed from the Internet without proper authorization, and the Internet itself is not considered to be a part of the intranet or extranet, although the Internet may serve as a portal for access to portions of an extranet.

Classification of computer networks

By network layer

Computer networks may be classified according to the network layer at which they operate according to some basic reference models that are considered to be standards in the industry such as the seven layer OSI reference model and the five layer TCP/IP model.

By scale

Computer networks may be classified according to the scale or extent of reach of the network, for example as a Personal area network (PAN), Local area network (LAN), Wireless local area network(WLAN), Campus area network (CAN), Metropolitan area network (MAN), or Wide area network (WAN).

By connection method

Computer networks may be classified according to the technology that is used to connect the individual devices in the network such as HomePNA, Power line communication, Ethernet, or WiFi.

By functional relationship

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

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 topology network, etc.

By Services provided

Computer networks may be classified according to the services which they provide, such as Storage area networks, Server farms, Process control networks, Value-added network, SOHO network, Wireless community network, XML appliance, Jungle Networks, khadar network, etc.

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.

See also

• Computer networking
• Computer networking device
• Node (networking)
• Network topology
• Bus network
• Mesh network
• Ring network
• Star network
• Tree and hypertree networks
• Expander graph
• Scale-free network
• Network diagram