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{{Network Science}}

A '''computer network''', or simply a '''network''', is a collection of [[computer]]s and other [[Networking hardware|hardware]] interconnected by communication channels that allow sharing of resources and information.<ref>{{citation |url=http://www.atis.org/glossary/definition.aspx?id=6555 |title=Computer network definition |accessdate=2011-11-12}}</ref> Where at least one process in one device is able to send/receive data to/from at least one process residing in a remote device, then the two devices are said to be in a network.
A network is a group of devices connected to each other. Networks may be classified into a wide variety of characteristics: the medium used to transport the data, [[communications protocol]] used, scale, [[Network topology|topology]], benefit, and organizational scope.

Communication protocols define the rules and data formats for exchanging information in a computer network, and provide the basis for [[computer network programming|network programming]]. Well-known communications protocols include two [[Ethernet]], a hardware and [[link layer]] standard that is ubiquitous in [[local area network]]s, and the [[Internet protocol suite]], which defines a set of protocols for internetworking, i.e. for data communication between multiple networks, as well as host-to-host data transfer, and application-specific data transmission formats.

Computer networking is sometimes considered a sub-discipline of [[electrical engineering]], [[telecommunications]], [[computer science]], [[information technology]] or [[computer engineering]], since it relies upon the theoretical and practical application of these disciplines.

==History==
[[File:Distributed Processing.jpg|thumb|Distributed processing]]
{{Expand section|date=December 2010}}
Before the advent of computer networks that were based upon some type of telecommunications system, communication between calculation machines and early computers was performed by human users by carrying instructions between them. Many of the social behaviors seen in today's Internet were demonstrably present in the 19th century and arguably in even earlier networks using visual signals.

* In September 1940, [[George Stibitz]] used a [[Teletype machine]] to send instructions for a problem set from his Model at [[Dartmouth College]] to his [[Complex Number Calculator]] in New York and received results back by the same means. Linking output systems like [[teletypewriter]]s to computers was an interest at the [[Advanced Research Projects Agency]] (ARPA) when, in 1962, [[J.C.R. Licklider]] was hired and developed a working group he called the "[[Intergalactic Computer Network]]", a precursor to the [[ARPANET]].
* Early networks of communicating computers included the military radar system [[Semi-Automatic Ground Environment]] (SAGE), started in the late 1950s.
* The commercial airline reservation system [[semi-automatic business research environment]] (SABRE) went online with two connected mainframes in 1960.
* In 1964, researchers at Dartmouth developed the [[Dartmouth Time Sharing System]] for distributed users of large computer systems. The same year, at [[Massachusetts Institute of Technology]], a research group supported by [[General Electric]] and [[Bell Labs]] used a computer to route and manage telephone connections.
* Throughout the 1960s [[Leonard Kleinrock]], [[Paul Baran]] and [[Donald Davies]] independently conceptualized and developed network systems which used [[packet (information technology)|packets]] that could be used in a network between computer systems.
* 1965 [[Thomas Marill (scientist)|Thomas Marill]] and [[Lawrence G. Roberts]] created the first [[wide area network]] (WAN). This was an immediate precursor to the [[ARPANET]], of which Roberts became program manager.
* The first widely used [[telephone switch]] that used true computer control was introduced by [[Western Electric]] in 1965.
* In 1969 the [[University of California at Los Angeles]], the [[Stanford Research Institute]], [[University of California at Santa Barbara]], and the [[University of Utah]] were connected as the beginning of the [[ARPANET]] network using 50 kbit/s circuits.<ref>{{cite web |title=Internet Began 35 Years Ago at UCLA with First Message Ever Sent Between Two Computers |url=http://www.engineer.ucla.edu/stories/2004/Internet35.htm |author=Chris Sutton|publisher=''[[UCLA]]''|archiveurl=http://web.archive.org/web/20080308120314/http://www.engineer.ucla.edu/stories/2004/Internet35.htm |archivedate=March 8, 2008}}</ref>
* Commercial services using [[X.25]] were deployed in 1972, and later used as an underlying infrastructure for expanding [[TCP/IP]] networks.

Today, computer networks are the core of modern communication. All modern aspects of the [[public switched telephone network]] (PSTN) are computer-controlled, and telephony increasingly runs over the Internet Protocol, although not necessarily the public Internet. The scope of communication has increased significantly in the past decade, and this boom in communications would not have been possible without the progressively advancing computer network. Computer networks and the technology that makes communication between other computers on those networks possible, continue to drive computer hardware, software, and peripherals industries. This expansion is mirrored by growth in the numbers and types of users of networks, from the researcher to the home user.

Interconnected collection of autonomous computers(unique identity) is known as computer network.

==Properties==
Computer networks:
; Facilitate communications : Using a network, people can communicate efficiently and easily via email, instant messaging, chat rooms, telephone, video telephone calls, and video conferencing.
; Permit sharing of files, data, and other types of information: In a network environment, authorized users may access data and information stored on other computers on the network. The capability of providing access to data and information on shared storage devices is an important feature of many networks.
; Share network and computing resources: In a networked environment, each computer on a network may access and use resources provided by devices on the network, such as printing a document on a shared network printer. [[Distributed computing]] uses computing resources across a network to accomplish tasks.

; May be insecure: A computer network may be used by [[Hacker (computer security)|computer hackers]] to deploy [[computer virus]]es or [[computer worm]]s on devices connected to the network, or to prevent these devices from normally accessing the network ([[Denial-of-service attack|denial of service]]).
; May interfere with other technologies: [[Power line communication]] strongly disturbs certain<ref>[[Andrew S Tanenbaum,4th Edition. Pearson Education/PHI.]]</ref>forms of radio communication, e.g., amateur radio. It may also interfere with [[last mile]] access technologies such as [[ADSL]] and [[VDSL]].
; May be difficult to set up: A complex computer network may be difficult to set up. It may also be very costly to set up an effective computer network in a large organization or company.

==Communication media==
Computer networks can be classified according to the hardware and associated software technology that is used to interconnect the individual devices in the network, such as [[Cable|electrical cable]] ([[HomePNA]], [[power line communication]], [[G.hn]]), [[optical fiber]], and [[radio waves]] ([[wireless LAN]]). In the [[OSI model]], these are located at levels 1 and 2.

A well-known ''family'' of communication media is collectively known as [[Ethernet]]. It is defined by [[IEEE 802]] and utilizes various standards and media that enable communication between devices. Wireless LAN technology is designed to connect devices without wiring. These devices use [[radio waves]] or [[IrDA|infrared]] signals as a transmission medium.

===Wired technologies===

The order of the following wired technologies are, roughly, from slowest to fastest transmission speed.

*''[[Twisted pair]] wire'' is the most widely used medium for telecommunication. Twisted-pair cabling consist of copper wires that are twisted into pairs. Ordinary telephone wires consist of two insulated copper wires twisted into pairs. Computer networking cabling (wired [[Ethernet]] as defined by [[IEEE 802.3]]) consists of 4 pairs of copper cabling that can be utilized for both voice and data transmission. The use of two wires twisted together helps to reduce [[crosstalk (electronics)|crosstalk]] and [[electromagnetic induction]]. The transmission speed ranges from 2 million bits per second to 10 billion bits per second. Twisted pair cabling comes in two forms: unshielded twisted pair (UTP) and shielded twisted-pair (STP). Each form comes in several category ratings, designed for use in various scenarios.

*''[[Coaxial cable]]'' is widely used for cable television systems, office buildings, and other work-sites for local area networks. The cables consist of copper or aluminum wire surrounded by an insulating layer (typically a flexible material with a high dielectric constant), which itself is surrounded by a conductive layer. The insulation helps minimize interference and distortion. Transmission speed ranges from 200 million bits per second to more than 500 million bits per second.

*[[ITU-T]] [[G.hn]] technology uses existing [[home wiring]] ([[Ethernet over coax|coaxial cable]], phone lines and [[Power line communication|power lines]]) to create a high-speed (up to 1 Gigabit/s) local area network.

*An [[optical fiber]] is a glass fiber. It uses pulses of light to transmit data. Some advantages of optical fibers over metal wires are less transmission loss, immunity from electromagnetic radiation, and very fast transmission speed, up to trillions of bits per second. One can use different colors of lights to increase the number of messages being sent over a fiber optic cable.

===Wireless technologies===
*''Terrestrial [[microwave]]''&nbsp;– Terrestrial microwave communication uses Earth-based transmitters and receivers resembling satellite dishes. Terrestrial microwaves are in the low-gigahertz range, which limits all communications to line-of-sight. Relay stations are spaced approximately {{convert|48|km|mi|abbr=on}} apart.

*''Communications [[satellite]]s''&nbsp;– The satellites communicate via microwave radio waves, which are not deflected by the Earth's atmosphere. The satellites are stationed in space, typically in geosynchronous orbit {{convert|35,400|km|mi|abbr=on}} above the equator. These Earth-orbiting systems are capable of receiving and relaying voice, data, and TV signals.

*''Cellular and PCS systems'' use several radio communications technologies. The systems divide the region covered into multiple geographic areas. Each area has a low-power transmitter or radio relay antenna device to relay calls from one area to the next area.

*''Radio and spread spectrum technologies''&nbsp;– Wireless local area network use a high-frequency radio technology similar to digital cellular and a low-frequency radio technology. Wireless LANs use spread spectrum technology to enable communication between multiple devices in a limited area. [[IEEE 802.11]] defines a common flavor of open-standards wireless radio-wave technology.

*[[Infrared communication]] can transmit signals for small distances, typically no more than 10 meters. In most cases, [[line-of-sight propagation]] is used, which limits the physical positioning of communicating devices.

*A [[global area network]] (GAN) is a network used for supporting mobile across an arbitrary number of wireless LANs, satellite coverage areas, etc. The key challenge in mobile communications is handing off user communications from one local coverage area to the next. In IEEE Project 802, this involves a succession of terrestrial [[wireless LAN]]s.<ref>{{cite web |url=http://grouper.ieee.org/groups/802/20/ |title=Mobile Broadband Wireless connections (MBWA) |accessdate=2011-11-12}}</ref>

===Exotic technologies===
There have been various attempts at transporting data over exotic media:

* [[IP over Avian Carriers]] was a humorous April fool's [[Request for Comments]], issued as '''RFC 1149'''. It was implemented in real life in 2001.<ref>[http://www.blug.linux.no/rfc1149 Bergen Linux User Group's CPIP Implementation]</ref>

* Extending the Internet to interplanetary dimensions via radio waves.<ref>{{citation |url=http://www.ipnsig.org/reports/ISART9-2000.pdf |title=Interplanetary Internet |publisher=Third Annual International Symposium on Advanced Radio Technologies |author=A. Hooke |date=September 2000 |accessdate=2011-11-12}}</ref>

Both cases have a large [[round-trip delay time]], which prevents useful communication.

==Communications protocols and network programming==
[[File:Internet map 1024.jpg|thumb|[[Internet]] map. The Internet is a global system of interconnected computer networks that use the [[Internet Protocol Suite|standard Internet Protocol Suite]] (TCP/IP) to serve billions of users worldwide.]]

A [[communications protocol]] is a set of rules for exchanging information over a network. It is typically a [[protocol stack]] (also see the [[OSI model]]), which is a "stack" of protocols, in which each protocol uses the protocol below it. An important example of a protocol stack is [[HTTP]] running over [[transmission control protocol|TCP]] over [[Internet protocol|IP]] over [[IEEE 802.11]] (TCP and IP are members of the [[Internet Protocol Suite]], and IEEE 802.11 is a member of the [[Ethernet]] protocol suite). This stack is used between the [[wireless router]] and the home user's personal computer when the user is surfing the web.

Communication protocols have various properties, such as whether they are [[Connection-oriented communication|connection-oriented]] or [[Connectionless communication|connectionless]], whether they use [[circuit mode]] or [[packet switching]], or whether they use hierarchical or flat addressing.

There are many communication protocols, a few of which are described below.

===Ethernet===
[[Ethernet]] is a family of protocols used in LANs, described by a set of standards together called [[IEEE 802]] published by the [[Institute of Electrical and Electronics Engineers]]. It has a flat addressing scheme and is mostly situated at levels 1 and 2 of the [[OSI model]]. For home users today, the most well-known member of this protocol family is [[IEEE 802.11]], otherwise known as [[Wireless LAN]] (WLAN). However, the complete protocol suite deals with a multitude of networking aspects not only for home use, but especially when the technology is deployed to support a diverse range of business needs. [[Media access control|MAC]] [[Bridging (networking)|bridging]] ([[IEEE 802.1D]]) deals with the routing of Ethernet packets using a [[Spanning Tree Protocol]], [[IEEE 802.1Q]] describes [[Virtual LAN|VLANs]], and [[IEEE 802.1X]] defines a port-based [[Network Access Control]] protocol, which forms the basis for the authentication mechanisms used in VLANs, but it is also found in WLANs&nbsp;– it is what the home user sees when the user has to enter a "wireless access key".

===Internet Protocol Suite===
The [[Internet Protocol Suite]], often also called TCP/IP, is the foundation of all modern internetworking. It offers connection-less as well as connection-oriented services over an inherently unreliable network traversed by datagram transmission at the [[Internet protocol]] (IP) level. At its core, the protocol suite defines the addressing, identification, and routing specification in form of the traditional [[IPv4|Internet Protocol Version 4]] (IPv4) and IPv6, the next generation of the protocol with a much enlarged addressing capability.

===SONET/SDH===
[[Synchronous optical networking]] (SONET) and Synchronous Digital Hierarchy (SDH) are standardized [[multiplexing]] protocols that transfer multiple digital bit streams over optical fiber using lasers. They were originally designed to transport circuit mode communications from a variety of different sources, primarily to support real-time, uncompressed, [[Circuit switching|circuit-switched]] voice encoded in [[Pulse code modulation|PCM]](Pulse-Code Modulation) format. However, due to its protocol neutrality and transport-oriented features, SONET/SDH also was the obvious choice for transporting [[Asynchronous Transfer Mode]] (ATM) frames.

===Asynchronous Transfer Mode===
[[Asynchronous transfer mode]] (ATM) is a switching technique for telecommunication networks. It uses asynchronous [[time-division multiplexing]] and encodes data into small, fixed-sized [[cell relay|cells]]. This differs from other protocols such as the [[Internet Protocol Suite]] or [[Ethernet]] that use variable sized packets or [[Frame Relay|frames]]. ATM has similarity with both [[Circuit switching|circuit]] and [[Packet switching|packet]] switched networking. This makes it a good choice for a network that must handle both traditional high-throughput data traffic, and real-time, [[Latency (engineering)|low-latency]] content such as voice and video. ATM uses a [[connection-oriented]] model in which a [[virtual circuit]] must be established between two endpoints before the actual data exchange begins.

While the role of ATM is diminishing in favor of [[Next generation network|next-generation networks]], it still plays a role in the [[last mile]], which is the connection between an [[Internet service provider]] and the home user. For an interesting write-up of the technologies involved, including the deep stacking of communications protocols used, see.<ref>{{cite web|last=Martin|first=Thomas|title=Design Principles for DSL-Based Access Solutions|url=http://www.gsi.dit.upm.es/~legf/Varios/XDSL_MARTI.PDF|accessdate=18 June 2011}}</ref>

===Network programming===
[[Computer network programming]] involves writing computer programs that communicate with each other across a computer network. Different programs must be written for the [[client (computing)|client]] process, which initiates the communication, and for the [[server (computing)|server]] process, which waits for the communication to be initiated. Both endpoints of the communication flow are implemented as [[network sockets]]; hence network programming is basically socket programming.

==Scale==
{{Area networks}}
Networks are often classified by their physical or organizational extent or their purpose. Usage, trust level, and access rights differ between these types of networks.

===Personal area network===
A [[personal area network]] (PAN) is a computer network used for communication among computer and different information technological devices close to one person. Some examples of devices that are used in a PAN are personal computers, printers, fax machines, telephones, PDAs, scanners, and even video game consoles. A PAN may include wired and wireless devices. The reach of a PAN typically extends to 10 meters.<ref>{{cite web |url=http://searchmobilecomputing.techtarget.com/sDefinition/0,,sid40_gci546288,00.html |title=personal area network (PAN) |accessdate=January 29, 2011}}</ref> A wired PAN is usually constructed with USB and Firewire connections while technologies such as Bluetooth and infrared communication typically form a wireless PAN.

===Local area network===
A [[local area network]] (LAN) is a network that connects computers and devices in a limited geographical area such as home, school, computer laboratory, office building, or closely positioned group of buildings. Each computer or device on the network is a node. Current wired LANs are most likely to be based on [[Ethernet]] technology, although new standards like [[ITU-T]] [[G.hn]] also provide a way to create a wired LAN using existing home wires (coaxial cables, phone lines and power lines).<ref>{{citation |url=http://www.itu.int/ITU-T/newslog/New+Global+Standard+For+Fully+Networked+Home.aspx |title=New global standard for fully networked home |publisher=ITU-T |accessdate=2011-11-12 |date=2008-12-12}}</ref>

[[File:NETWORK-Library-LAN.png|thumb | Typical library network, in a branching tree topology and controlled access to resources]]

A sample LAN is depicted in the accompanying diagram. 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 [[Internet Protocol|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 no need for leased telecommunication lines. Current Ethernet or other [[IEEE 802.3]] LAN technologies operate at data transfer rates up to 10 Gbit/s. [[IEEE]] has projects investigating the standardization of 40 and 100 Gbit/s.<ref>{{citation |url=http://www.ieee802.org/3/ba/ |title=IEEE P802.3ba 40Gb/s and 100Gb/s Ethernet Task Force |accessdate=2011-11-12}}</ref>
LANs can be connected to Wide area network by using routers.

===Home area network===
A [[home area network]] (HAN) is a residential LAN which is used for communication between digital devices typically deployed in the home, usually a small number of personal computers and accessories, such as printers and mobile computing devices. An important function is the sharing of Internet access, often a broadband service through a cable TV or [[Digital Subscriber Line]] (DSL) provider.

===Storage area network===
A [[storage area network]] (SAN) is a dedicated network that provides access to consolidated, block level data storage. SANs are primarily used to make storage devices, such as disk arrays, tape libraries, and optical jukeboxes, accessible to servers so that the devices appear like locally attached devices to the operating system. A SAN typically has its own network of storage devices that are generally not accessible through the local area network by other devices. The cost and complexity of SANs dropped in the early 2000s to levels allowing wider adoption across both enterprise and small to medium sized business environments.

===Campus area network===
A [[campus area network]] (CAN) is a computer network made up of an interconnection of LANs within a limited geographical area. The networking equipment (switches, routers) and transmission media (optical fiber, copper plant, [[Category 5 cable|Cat5]] cabling etc.) are almost entirely owned (by the campus tenant / owner: an enterprise, university, government etc.).

In the case of a university campus-based campus network, the network is likely to link a variety of campus buildings including, for example, academic colleges or departments, the university library, and student residence halls.

===Backbone network===
A [[backbone network]] is part of a computer network infrastructure that interconnects various pieces of network, providing a path for the exchange of information between different LANs or subnetworks. A backbone can tie together diverse networks in the same building, in different buildings in a campus environment, or over wide areas. Normally, the backbone's capacity is greater than that of the networks connected to it.

A large corporation which has many locations may have a backbone network that ties all of these locations together. For example, a server cluster that needs to be accessed by different departments of a company, which are located around the world, would benefit from the use of a backbone network. The equipment which ties these departments together constitute the network backbone. [[Network performance management]] including [[network congestion]] are critical parameters taken into account when designing a network backbone.

A specific case of a backbone network is the [[Internet backbone]], which is the set of wide-area network connections and [[core router]]s that interconnect all networks connected to the [[Internet]].

===Metropolitan area network===
A [[Metropolitan area network]] (MAN) is a large computer network that usually spans a city or a large campus.
[[File:EPN Frame-Relay and Dial-up Network.svg|thumb|upright=1.5| Sample EPN made of [[Frame relay]] WAN connections and dialup remote access.]]

[[File:Virtual Private Network overview.svg|thumb|upright=1.5|Sample VPN used to interconnect 3 offices and remote users]]

===Wide area network===
A [[wide area network]] (WAN) is a computer network that covers a large geographic area such as a city, country, or spans even intercontinental distances, using a communications channel that combines many types of media such as telephone lines, cables, and air waves. A WAN often uses transmission facilities provided by common carriers, such as telephone companies. WAN technologies generally function at the lower three layers of the [[OSI model|OSI reference model]]: the [[physical layer]], the [[data link layer]], and the [[network layer]].

===Enterprise private network===
An [[enterprise private network]] is a network built by an enterprise to interconnect various company sites, e.g., production sites, head offices, remote offices, shops, in order to share computer resources.

===Virtual private network===
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 data 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.

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.

===Virtual Network===
Not to be confused with a [[Virtual Private Network]], a Virtual Network defines data traffic flows between [[virtual machines]] within a [[hypervisor]] in a virtual computing environment. Virtual Networks may employ [[virtual security switch]]es, [[virtual router]]s, [[virtual firewall]]s and other virtual networking devices to direct and secure data traffic.

===Internetwork===
An [[internetwork]] is the connection of multiple computer networks via a common routing technology using routers. The [[Internet]] is an aggregation of many connected internetworks spanning the [[Earth]].

==Organizational scope==
Networks are typically managed by organizations which own them. According to the owner's point of view, networks are seen as intranets or extranets. A special case of network is the [[Internet]], which has no single owner but a distinct status when seen by an organizational entity&nbsp;– that of permitting virtually unlimited global connectivity for a great multitude of purposes.

===Intranets and extranets===
Intranets and extranets are parts or extensions of a computer network, usually a LAN.

An [[intranet]] is a set of networks, using the [[Internet Protocol]] and IP-based tools such as web browsers and file transfer applications, that are 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.

An [[extranet]] is a network that is limited in scope to a single organization or entity and also has limited connections to the networks of one or more other usually, but not necessarily, trusted organizations or entities—a company's customers may be given access to some part of its intranet—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 an extranet cannot consist of a single LAN; it must have at least one connection with an external network.

===Internet===
The Internet is a global system of interconnected governmental, academic, corporate, public, and private computer networks. It is based on the networking technologies of the [[Internet Protocol Suite]]. It is the successor of the [[ARPANET|Advanced Research Projects Agency Network]] (ARPANET) developed by [[Defense Advanced Research Projects Agency|DARPA]] of the [[United States Department of Defense]]. The Internet is also the communications backbone underlying the [[World Wide Web]] (WWW).

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 address]]es) administered by the [[Internet Assigned Numbers Authority]] and [[Regional Internet Registry|address registries]]. Service providers and large enterprises exchange information about the [[routing|reachability]] of their address spaces through the [[Border Gateway Protocol]] (BGP), forming a redundant worldwide mesh of transmission paths.

==Network topology==
===Common layouts===
A [[network topology]] is the layout of the interconnections of the nodes of a computer network. Common layouts are:
* A [[bus network]]: all nodes are connected to a common medium along this medium. This was the layout used in the original [[Ethernet]], called [[10BASE5]] and [[10BASE2]].
* A [[star network]]: all nodes are connected to a special central node. This is the typical layout found in a [[Wireless LAN]], where each wireless client connects to the central [[Wireless access point]].
* A [[ring network]]: each node is connected to its left and right neighbour node, such that all nodes are connected and that each node can reach each other node by traversing nodes left- or rightwards. The [[Fiber Distributed Data Interface]] (FDDI) made use of such a topology.
* A [[mesh network]]: each node is connected to an arbitrary number of neighbours in such a way that there is at least one traversal from any node to any other.
* A fully connected network: each node is connected to every other node in the network.

Note that the physical layout of the nodes in a network may not necessarily reflect the network topology. As an example, with [[FDDI]], the network topology is a ring (actually two counter-rotating rings), but the physical topology is a star, because all neighboring connections are routed via a central physical location.

===Overlay network===
An [[overlay network]] is a virtual computer network that is built on top of another network. Nodes in the overlay are connected by virtual or logical links, each of which corresponds to a path, perhaps through many physical links, in the underlying network. The topology of the overlay network may (and often does) differ from that of the underlying one.
[[File:Network Overlay.svg|thumb|upright=1.5| A sample overlay network: IP over SONET over Optical]] For example, many [[peer-to-peer]] networks are overlay networks because they are organized as nodes of a virtual system of links run on top of the Internet. The Internet was initially built as an overlay on the [[telephone network]].<ref>{{citation |author1=D. Andersen |author2=H. Balakrishnan |author3=M. Kaashoek |author4=[[Robert Tappan Morris|R. Morris]] |url=http://nms.lcs.mit.edu/papers/ron-sosp2001.html |title=Resilient Overlay Networks] |date=10-2001 |publisher=[[Association for Computing Machinery]] |accessdate=2011-11-12}}</ref>

The most striking example of an overlay network, however, is the Internet itself: At the network layer, each node can reach any other by a direct connection to the desired IP address, thereby creating a fully connected network; the underlying network, however, is composed of a mesh-like interconnect of subnetworks of varying topologies (and, in fact, technologies). [[Address Resolution Protocol|Address resolution]] and [[routing]] are the means which allows the mapping of the fully connected IP overlay network to the underlying ones.

Overlay networks have been around since the invention of networking when computer systems were connected over telephone lines using [[modem]]s, before any data network existed.

Another example of an overlay network is a [[distributed hash table]], which maps keys to nodes in the network. In this case, the underlying network is an IP network, and the overlay network is a table (actually a [[associative array|map]]) indexed by keys.

Overlay networks have also been proposed as a way to improve Internet routing, such as through [[quality of service]] guarantees to achieve higher-quality [[streaming media]]. Previous proposals such as [[IntServ]], [[DiffServ]], and [[IP Multicast]] have not seen wide acceptance largely because they require modification of all [[Router (computing)|router]]s in the network.{{Citation needed|date=August 2010}} On the other hand, an overlay network can be incrementally deployed on end-hosts running the overlay protocol software, without cooperation from [[Internet service provider]]s. The overlay has no control over how packets are routed in the underlying network between two overlay nodes, but it can control, for example, the sequence of overlay nodes a message traverses before reaching its destination.

For example, [[Akamai Technologies]] manages an overlay network that provides reliable, efficient content delivery (a kind of [[multicast]]). Academic research includes [http://esm.cs.cmu.edu/ end system multicast] and overcast for multicast; RON ([[resilient overlay network]]) for resilient routing; and OverQoS for quality of service guarantees, among others.

==Basic hardware components==
{{main|Networking hardware}}
Apart from the physical communications media themselves as described above, networks comprise additional basic hardware building blocks interconnecting their terminals, such as [[Network interface controller|network interface cards (NICs)]], [[Ethernet hub|hubs]], [[Network bridge|bridges]], [[Network switch|switches]], and [[Router (computing)|router]]s.

===Network interface cards===
A [[network card]], network adapter, or NIC (network interface card) is a piece of [[computer hardware]] designed to allow computers to physically access a networking medium. It provides a low-level addressing system through the use of [[MAC address]]es.

Each [[Ethernet]] network interface has a unique MAC address which is usually stored in a small memory device on the card, allowing any device to connect to the network without creating an address conflict. Ethernet MAC addresses are composed of six [[Octet (computing)|octets]]. Uniqueness is maintained by the [[Institute of Electrical and Electronics Engineers|IEEE]], which manages the Ethernet address space by assigning 3-octet prefixes to equipment manufacturers. The [http://standards.ieee.org/regauth/oui/oui.txt list of prefixes] is publicly available. Each manufacturer is then obliged to both use only their assigned prefix(es) and to uniquely set the 3-octet suffix of every Ethernet interface they produce.

===Repeaters and hubs===
A [[repeater]] is an [[Electronics|electronic]] device that receives a [[signal (information theory)|signal]], cleans it of unnecessary noise, regenerates it, and [[retransmission (data networks)|retransmit]]s 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 that runs longer than 100 meters. A repeater with multiple ports is known as a [[Network hub|hub]]. Repeaters work on the physical layer of the OSI model. Repeaters require a small amount of time to regenerate the signal. This can cause a [[propagation delay]] which can affect network communication when there are several repeaters in a row. Many network architectures limit the number of repeaters that can be used in a row (e.g. Ethernet's [[5-4-3 rule]]).

Today, repeaters and hubs have been made mostly obsolete by switches (see below).

===Bridges===
A [[network bridge]] connects multiple [[network segment]]s at the [[data link layer]] (layer 2) of the [[OSI model]]. Bridges broadcast to all ports except the port on which the broadcast was received. However, bridges do not promiscuously copy traffic to all ports, as hubs do, but learn which [[MAC Address|MAC addresses]] are reachable through specific ports. Once the bridge associates a port and an address, it will send traffic for that address to that port only.

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 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 with routers.
* Wireless bridges: Can be used to join LANs or connect remote stations to LANs.

===Switches===
A [[network switch]] is a device that forwards and filters [[OSI layer 2]] [[datagrams]] between [[Computer port (hardware)|ports]] based on the MAC addresses in the packets.<ref>
{{cite web
|url=http://www.webopedia.com/TERM/s/switch.html
|title=Define switch.
|publisher=WWW.Wikipedia.com
|accessdate=April 8, 2008
}}</ref>
A switch is distinct from a hub in that it only forwards the frames to the ports involved in the communication rather than all ports connected. A switch breaks the collision domain but represents itself as a broadcast domain. Switches make forwarding decisions of frames on the basis of MAC addresses. A switch normally has numerous ports, facilitating a star topology for devices, and cascading additional switches.
Some switches are capable of routing based on layer 3 addressing or additional logical levels; these are called multi-layer switches. The term ''switch'' is used loosely in marketing to encompass devices including routers and bridges, as well as devices that may distribute traffic on load or by application content (e.g., a Web [[Uniform Resource Locator|URL]] identifier).

===Routers===
A [[Router (computing)|router]] is an internetworking device that forwards [[Packet (information technology)|packets]] between networks by processing information found in the datagram or packet (Internet protocol information from layer 3). In many situations, this information is processed in conjunction with the routing table (also known as forwarding table). Routers use routing tables to determine what interface to forward packets (this can include the "null" also known as the "black hole" interface because data can go into it, however, no further processing is done for said data).

===Firewalls===
A [[Firewall (computing)|firewall]] is an important aspect of a network with respect to security. It typically rejects access requests from unsafe sources while allowing actions from recognized ones. The vital role firewalls play in network security grows in parallel with the constant increase in 'cyber' attacks for the purpose of stealing/corrupting data, planting viruses, etc.'''

==Network performance==

{{main|network performance}}

'''Network performance''' refers to the [[service quality]] of a telecommunications product as seen by the customer. It should not be seen merely as an attempt to get "more through" the network.

The following list gives examples of Network Performance measures for a circuit-switched network and one type of [[packet-switched network]], viz. ATM:

*Circuit-switched networks: In [[circuit switched]] networks, network performance is synonymous with the [[grade of service]]. The number of rejected calls is a measure of how well the network is performing under heavy traffic loads.<ref>{{citation |url=http://www.com.dtu.dk/teletraffic/handbook/telenook.pdf |archiveurl=http://web.archive.org/web/20070111015452/http://oldwww.com.dtu.dk/teletraffic/handbook/telenook.pdf |archivedate=2007-01-11 |publisher=ITU-T Study Group 2 |title=Teletraffic Engineering Handbook}}</ref> Other types of performance measures can include noise, echo and so on.

*ATM: In an [[Asynchronous Transfer Mode]] (ATM) network, performance can be measured by line rate, [[quality of service]] (QoS), data throughput, connect time, stability, technology, modulation technique and modem enhancements.<ref>[http://www.telecommagazine.com Telecommunications Magazine Online], Americas January 2003, Issue Highlights, Online Exclusive: Broadband Access Maximum Performance, Retrieved on February 13, 2005.</ref>

There are many different ways to measure the performance of a network, as each network is different in nature and design. Performance can also be modelled instead of measured; one example of this is using state transition diagrams to model queuing performance in a circuit-switched network. These diagrams allow the network planner to analyze how the network will perform in each state, ensuring that the network will be optimally designed.<ref>{{cite web|url=http://cne.gmu.edu/modules/os_perf/std.t.html |title = State Transition Diagrams |accessdate =July 13, 2003}}</ref>

==Network security==

{{main|network security}}

In the field of networking, the area of '''network security'''<ref>{{cite journal | doi = 10.1007/978-3-540-30176-9_41 | last = Simmonds | first = A | coauthors = Sandilands, P; van Ekert, L|title = An Ontology for Network Security Attacks | journal = Lecture Notes in Computer Science | volume = 3285 | pages = 317–323 | year = 2004 | series = Lecture Notes in Computer Science | isbn = 978-3-540-23659-7 }}</ref> consists of the provisions and [[policies]] adopted by the [[network administrator]] to prevent and monitor [[unauthorized]] access, misuse, modification, or denial of the computer network and network-accessible resources. Network security is the authorization of access to data in a network, which is controlled by the network administrator. Users are assigned an ID and password that allows them access to information and programs within their authority. Network Security covers a variety of computer networks, both public and private that are used in everyday jobs conducting transactions and communications among businesses, government agencies and individuals.

==Network resilience==

{{main|resilience (network)}}

In computer networking: “'''Resilience''' is the ability to provide and maintain an acceptable level of [[Service (systems architecture)|service]] in the face of [[Fault (technology)|faults]] and challenges to normal operation.”<ref>{{cite web |url=http://wiki.ittc.ku.edu/resilinets_wiki/index.php/Definitions#Resilience |publisher=ResiliNets Research Initiative |title=Definitions: Resilience |accessdate=2011-11-12}}</ref>

==Views of networks==

Users and network administrators typically have different views of their networks. Users can share printers and some servers from a workgroup, which usually means they are in the same geographic location and are on the same LAN, whereas a Network Administrator is responsible to keep that network up and running. A [[community-of-interest network|community of interest]] has less of a connection of being in a local area, and should be thought of as a set of arbitrarily located users who share a set of servers, and possibly also communicate via [[peer-to-peer]] technologies.

Network administrators can see networks from both physical and logical perspectives. The physical perspective involves geographic locations, physical cabling, and the network elements (e.g., [[Router (computing)|router]]s, [[Network bridge|bridges]] and [[Application-level gateway|application layer gateways]]) that interconnect the physical media. Logical networks, called, in the TCP/IP architecture, [[subnetwork|subnets]], map onto one or more physical media. For example, a common practice in a campus of buildings is to make a set of LAN cables in each building appear to be a common subnet, using [[Virtual LAN| virtual LAN (VLAN)]] technology.

Both users and administrators will be aware, to varying extents, of the trust and scope characteristics of a network. Again using TCP/IP architectural terminology, an [[intranet]] is a community of interest under private administration usually by an enterprise, and is only accessible by authorized users (e.g. employees).<ref name="RFC2547">RFC 2547</ref> Intranets do not have to be connected to the Internet, but generally have a limited connection. An [[extranet]] is an extension of an intranet that allows secure communications to users outside of the intranet (e.g. business partners, customers).<ref name="RFC2547"/>

Unofficially, the Internet is the set of users, enterprises, and content providers that are interconnected by [[Internet Service Providers]] (ISP). From an engineering viewpoint, the [[Internet]] is the set of subnets, and aggregates of subnets, which share the registered [[IP address]] space and exchange information about the reachability of those IP addresses using the [[Border Gateway Protocol]]. Typically, the [[human-readable]] names of servers are translated to IP addresses, transparently to users, via the directory function of the [[Domain Name System]] (DNS).

Over the Internet, there can be [[Business-to-business| business-to-business (B2B)]], [[Business-to-consumer| business-to-consumer (B2C)]] and [[Consumer-to-consumer electronic commerce|consumer-to-consumer (C2C)]] communications. Especially when money or sensitive information is exchanged, the communications are apt to be '''secured''' by some form of [[communications security]] mechanism. Intranets and extranets can be securely superimposed onto the Internet, without any access by general Internet users and administrators, using secure [[Virtual Private Network]] (VPN) technology.

==See also==
*[[Comparison of network diagram software]]
*[[Network topology]]
*[[Network Simulator]]

==References==
{{Reflist|30em}}
{{FS1037C}}

==Further reading==
* Shelly, Gary, et al. "Discovering Computers" 2003 Edition
* Cisco Systems, Inc., (2003, March 14). CCNA: network media types. Retrieved from [http://www.ciscopress.com/articles/article.asp?p=31276&rll=1 ciscopress.com]
* Wendell Odom,Rus Healy, Denise Donohue. (2010) CCIE Routing and Switching. Indianapolis, IN: Cisco Press
* Kurose James F and Keith W. Ross : Computer Networking: A Top-Down Approach Featuring the Internet, Pearson Education 2005.
* [[Andrew S. Tanenbaum]], ''Computer Networks'', Fourth Edition, Pearson Education 2006 (ISBN 0-13-349945-6).
* [[William Stallings]], ''Computer Networking with Internet Protocols and Technology'', Pearson Education 2004.
* [[List of important publications in computer science#Computer networks|Important publications in computer networks]]
* [[Vinton G. Cerf]] [http://www.cs.washington.edu/homes/lazowska/cra/networks.html "Software: Global Infrastructure for the 21st Century"]
* Meyers, Mike, "Mike Meyers' Certification Passport: Network+" ISBN 0-07-225348-7"
* Odom, Wendall, "CCNA Certification Guide"
* Network Communication Architecture and Protocols: OSI Network Architecture 7 Layers Model
* Bonaventure Olivier, [http://inl.info.ucl.ac.be/cnp3 "Computer Networking : Principles, Protocols and Practice"], online ebook, 2010

==External links==
* {{dmoz|Computers/Software/Networking/|Networking}}

{{Telecommunications}}
{{Operating System}}
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