computer network datacom redirects here for other uses see datacom disambiguation network science theory graph complex network contagion smallworld scalefree community structure percolation evolution controllability graph drawing social capital link analysis optimization reciprocity closure homophily transitivity preferential attachment balance theory network effect social influence network types informational computing telecommunication transport social scientific collaboration biological artificial neural interdependent semantic spatial dependency flow onchip graphs features clique component cut cycle data structure edge loop neighborhood path vertex adjacency list matrix incidence list matrix types bipartite complete directed hyper multi random weighted metricsalgorithms centrality degree betweenness closeness pagerank motif clustering degree distribution assortativity distance modularity efficiency models topology random graph erdsrnyi barabsialbert fitness model wattsstrogatz exponential random ergm random geometric rgg hyperbolic hgn hierarchical stochastic block model lancichinettifortunatoradicchi benchmark dynamics boolean network agent based epidemicsir listscategories topics software network scientists categorynetwork theory categorygraph theory vte operating systems common features process management interrupts memory management file system device drivers networking security io vte a computer network or data network is a digital telecommunications network which allows nodes to share resources in computer networks computing devices exchange data with each other using connections data links between nodes these data links are established over cable media such as wires or optic cables or wireless media such as wifi network computer devices that originate route and terminate the data are called network nodes nodes are identified by network addresses and can include hosts such as personal computers phones servers as well as networking hardware two such devices can be said to be networked together when one device is able to exchange information with the other device whether or not they have a direct connection to each other in most cases applicationspecific communications protocols are layered ie carried as payload over other more general communications protocols this formidable collection of information technology requires skilled network management to keep it all running reliably computer networks support an enormous number of applications and services such as access to the world wide web digital video digital audio shared use of application and storage servers printers and fax machines and use of email and instant messaging applications as well as many others computer networks differ in the transmission medium used to carry their signals communications protocols to organize network traffic the networks size topology traffic control mechanism and organizational intent the bestknown computer network is the internet history see also history of the internet the chronology of significant computernetwork developments includes in the late 1950s early networks of computers included the us military radar system semiautomatic ground environment sage in 1959 anatolii ivanovich kitov proposed to the central committee of the communist party of the soviet union a detailed plan for the reorganisation of the control of the soviet armed forces and of the soviet economy on the basis of a network of computing centres the ogas in 1960 the commercial airline reservation system semiautomatic business research environment sabre went online with two connected mainframes in 1963 j c r licklider sent a memorandum to office colleagues discussing the concept of the intergalactic computer network a computer network intended to allow general communications among computer users in 1964 researchers at dartmouth college 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 paul baran and donald davies independently developed the concept of packet switching to transfer information between computers over a network davies pioneered the implementation of the concept with the npl network a local area network at the national physical laboratory united kingdom using a line speed of 768 kbits in 1965 western electric introduced the first widely used telephone switch that implemented true computer control in 1966 thomas marill and lawrence g roberts published a paper on an experimental wide area network wan for computer time sharing in 1969 the first four nodes of the arpanet were connected using 50kbits circuits between the university of california at los angeles the stanford research institute the university of california at santa barbara and the university of utah his theoretical work on hierarchical routing in the late 1970s with student farouk kamoun remains critical to the operation of the internet today in 1972 commercial services using x25 were deployed and later used as an underlying infrastructure for expanding tcpip networks in 1973 the french cyclades network was the first to make the hosts responsible for the reliable delivery of data rather than this being a centralized service of the network itself in 1973 robert metcalfe wrote a formal memo at xerox parc describing ethernet a networking system that was based on the aloha network developed in the 1960s by norman abramson and colleagues at the university of hawaii in july 1976 robert metcalfe and david boggs published their paper ethernet distributed packet switching for local computer networks in 1976 john murphy of datapoint corporation created arcnet a tokenpassing network first used to share storage devices in 1995 the transmission speed capacity for ethernet increased from 10mbits to 100mbits by 1998 ethernet supported transmission speeds of a gigabit subsequently higher speeds of up to 400gbits were added as of 2018 properties computer networking may be considered a branch of electrical engineering electronics engineering telecommunications computer science information technology or computer engineering since it relies upon the theoretical and practical application of the related disciplines a computer network facilitates interpersonal communications allowing users to communicate efficiently and easily via various means email instant messaging online chat telephone video telephone calls and video conferencing a network allows sharing of network and computing resources users may access and use resources provided by devices on the network such as printing a document on a shared network printer or use of a shared storage device a network allows sharing of files data and other types of information giving authorized users the ability to access information stored on other computers on the network distributed computing uses computing resources across a network to accomplish tasks a computer network may be used by security hackers to deploy computer viruses or computer worms on devices connected to the network or to prevent these devices from accessing the network via a denialofservice attack network packet network packet computer communication links that do not support packets such as traditional pointtopoint telecommunication links simply transmit data as a bit stream however most information in computer networks is carried in packets a network packet is a formatted unit of data a list of bits or bytes usually a few tens of bytes to a few kilobytes long carried by a packetswitched network packets are sent through the network to their destination once the packets arrive they are reassembled into their original message packets consist of two kinds of data control information and user data payload the control information provides data the network needs to deliver the user data for example source and destination network addresses error detection codes and sequencing information typically control information is found in packet headers and trailers with payload data in between with packets the bandwidth of the transmission medium can be better shared among users than if the network were circuit switched when one user is not sending packets the link can be filled with packets from other users and so the cost can be shared with relatively little interference provided the link isnt overused often the route a packet needs to take through a network is not immediately available in that case the packet is queued and waits until a link is free network topology network topology the physical layout of a network is usually less important than the topology that connects network nodes most diagrams that describe a physical network are therefore topological rather than geographic the symbols on these diagrams usually denote network links and network nodes network links further information data transmission the transmission media often referred to in the literature as the physical media used to link devices to form a computer network include electrical cable optical fiber and radio waves in the osi model these are defined at layers 1 and 2 the physical layer and the data link layer a widely adopted family of transmission media used in local area network lan technology is collectively known as ethernet the media and protocol standards that enable communication between networked devices over ethernet are defined by ieee 8023 ethernet transmits data over both copper and fiber cables wireless lan standards use radio waves others use infrared signals as a transmission medium power line communication uses a buildings power cabling to transmit data wired technologies fiber optic cables are used to transmit light from one computernetwork node to another the following classes of wired technologies are used in computer networking coaxial cable is widely used for cable television systems office buildings and other worksites for local area networks transmission speed ranges from 200 million bits per second to more than 500 million bits per second itut ghn technology uses existing home wiring coaxial cable phone lines and power lines to create a highspeed local area network twisted pair cabling is used for wired ethernet and other standards it typically 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 and electromagnetic induction the transmission speed ranges from 2mbits to 10gbits twisted pair cabling comes in two forms unshielded twisted pair utp and shielded twistedpair stp each form comes in several category ratings designed for use in various scenarios 2007 map showing submarine optical fiber telecommunication cables around the world an optical fiber is a glass fiber it carries pulses of light that represent data some advantages of optical fibers over metal wires are very low transmission loss and immunity from electrical interference optical fibers can simultaneously carry multiple wavelengths of light which greatly increases the rate that data can be sent and helps enable data rates of up to trillions of bits per second optic fibers can be used for long runs of cable carrying very high data rates and are used for undersea cables to interconnect continents there are two types of transmission of fiber optics singlemode fiber smf and multimode fiber mmf singlemode fiber has the advantage of being able to sustain a coherent signal for dozens or even a hundred kilometers multimode fiber is cheaper to terminate but is limited to a few hundred or even only a few dozens of meters depending on the data rate and cable grade price is a main factor distinguishing wired and wirelesstechnology options in a business wireless options command a price premium that can make purchasing wired computers printers and other devices a financial benefit before making the decision to purchase hardwired technology products a review of the restrictions and limitations of the selections is necessary business and employee needs may override any cost considerations wireless technologies computers are very often connected to networks using wireless links wireless network terrestrial microwave terrestrial microwave communication uses earthbased transmitters and receivers resembling satellite dishes terrestrial microwaves are in the low gigahertz range which limits all communications to lineofsight relay stations are spaced approximately 48km 30mi apart communications satellites satellites communicate via microwave radio waves which are not deflected by the earths atmosphere the satellites are stationed in space typically in geosynchronous orbit 35400km 22000mi above the equator these earthorbiting 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 lowpower transmitter or radio relay antenna device to relay calls from one area to the next area radio and spread spectrum technologies wireless local area networks use a highfrequency radio technology similar to digital cellular and a lowfrequency radio technology wireless lans use spread spectrum technology to enable communication between multiple devices in a limited area ieee 80211 defines a common flavor of openstandards wireless radiowave technology known as wifi freespace optical communication uses visible or invisible light for communications in most cases lineofsight propagation is used which limits the physical positioning of communicating devices exotic technologies there have been various attempts at transporting data over exotic media ip over avian carriers was a humorous april fools request for comments issued as rfc1149 it was implemented in real life in 2001 extending the internet to interplanetary dimensions via radio waves the interplanetary internet both cases have a large roundtrip delay time which gives slow twoway communication but doesnt prevent sending large amounts of information network nodes node networking apart from any physical transmission media there may be networks comprise additional basic system building blocks such as network interface controllers nics repeaters hubs bridges switches routers modems and firewalls any particular piece of equipment will frequently contain multiple building blocks and perform multiple functions network interfaces an atm network interface in the form of an accessory card a lot of network interfaces are builtin a network interface controller nic is computer hardware that provides a computer with the ability to access the transmission media and has the ability to process lowlevel network information for example the nic may have a connector for accepting a cable or an aerial for wireless transmission and reception and the associated circuitry the nic responds to traffic addressed to a network address for either the nic or the computer as a whole in ethernet networks each network interface controller has a unique media access control mac addressusually stored in the controllers permanent memory to avoid address conflicts between network devices the institute of electrical and electronics engineers ieee maintains and administers mac address uniqueness the size of an ethernet mac address is six octets the three most significant octets are reserved to identify nic manufacturers these manufacturers using only their assigned prefixes uniquely assign the three leastsignificant octets of every ethernet interface they produce repeaters and hubs a repeater is an electronic device that receives a network signal cleans it of unnecessary noise and regenerates it the signal is retransmitted 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 with fiber optics repeaters can be tens or even hundreds of kilometers apart a repeater with multiple ports is known as an ethernet 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 that affects network performance and may affect proper function as a result many network architectures limit the number of repeaters that can be used in a row eg the ethernet 543 rule hubs and repeaters in lans have been mostly obsoleted by modern switches bridges a network bridge connects and filters traffic between two network segments at the data link layer layer 2 of the osi model to form a single network this breaks the networks collision domain but maintains a unified broadcast domain network segmentation breaks down a large congested network into an aggregation of smaller more efficient networks 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 devices to lans switches a network switch is a device that forwards and filters osi layer 2 datagrams frames between ports based on the destination mac address in each frame a switch is distinct from a hub in that it only forwards the frames to the physical ports involved in the communication rather than all ports connected it can be thought of as a multiport bridge it learns to associate physical ports to mac addresses by examining the source addresses of received frames if an unknown destination is targeted the switch broadcasts to all ports but the source switches normally have numerous ports facilitating a star topology for devices and cascading additional switches routers a typical home or small office router showing the adsl telephone line and ethernet network cable connections a router is an internetworking device that forwards packets between networks by processing the routing information included in the packet or datagram internet protocol information from layer 3 the routing information is often processed in conjunction with the routing table or forwarding table a router uses its routing table to determine where to forward packets a destination in a routing table can include a null interface also known as the black hole interface because data can go into it however no further processing is done for said data ie the packets are dropped modems modems modulatordemodulator are used to connect network nodes via wire not originally designed for digital network traffic or for wireless to do this one or more carrier signals are modulated by the digital signal to produce an analog signal that can be tailored to give the required properties for transmission modems are commonly used for telephone lines using a digital subscriber line technology firewalls a firewall is a network device for controlling network security and access rules firewalls are typically configured to reject access requests from unrecognized 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 network structure network topology is the layout or organizational hierarchy of interconnected nodes of a computer network different network topologies can affect throughput but reliability is often more critical with many technologies such as bus networks a single failure can cause the network to fail entirely in general the more interconnections there are the more robust the network is but the more expensive it is to install common layouts common network topologies 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 this is still a common topology on the data link layer although modern physical layer variants use pointtopoint links instead 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 a tree network nodes are arranged hierarchically 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 counterrotating rings but the physical topology is often a star because all neighboring connections can be routed via a central physical location overlay network a sample overlay network an overlay network is a virtual computer network that is built on top of another network nodes in the overlay network are connected by virtual or logical links each link 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 for example many peertopeer networks are overlay networks they are organized as nodes of a virtual system of links that run on top of the internet overlay networks have been around since the invention of networking when computer systems were connected over telephone lines using modems before any data network existed the most striking example of an overlay network is the internet itself the internet itself was initially built as an overlay on the telephone network even today each internet node can communicate with virtually any other through an underlying mesh of subnetworks of wildly different topologies and technologies address resolution and routing are the means that allow mapping of a fully connected ip overlay network to its underlying network 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 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 higherquality streaming media previous proposals such as intserv diffserv and ip multicast have not seen wide acceptance largely because they require modification of all routers in the network on the other hand an overlay network can be incrementally deployed on endhosts running the overlay protocol software without cooperation from internet service providers the overlay network 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 that a message traverses before it reaches its destination for example akamai technologies manages an overlay network that provides reliable efficient content delivery a kind of multicast academic research includes end system multicast resilient routing and quality of service studies among others communication protocols the tcpip model or internet layering scheme and its relation to common protocols often layered on top of it figure 4 message flows ab in the presence of a router r red flows are effective communication paths black paths are across the actual network links a communication protocol is a set of rules for exchanging information over a network in a protocol stack also see the osi model each protocol leverages the services of the protocol layer below it until the lowest layer controls the hardware which sends information across the media the use of protocol layering is today ubiquitous across the field of computer networking an important example of a protocol stack is http the world wide web protocol running over tcp over ip the internet protocols over ieee 80211 the wifi protocol this stack is used between the wireless router and the home users personal computer when the user is surfing the web communication protocols have various characteristics they may be connectionoriented or connectionless they may use circuit mode or packet switching and they may use hierarchical addressing or flat addressing there are many communication protocols a few of which are described below ieee 802 ieee 802 is a family of ieee standards dealing with local area networks and metropolitan area networks the complete ieee 802 protocol suite provides a diverse set of networking capabilities the protocols have a flat addressing scheme they operate mostly at levels 1 and 2 of the osi model for example mac bridging ieee 8021d deals with the routing of ethernet packets using a spanning tree protocol ieee 8021q describes vlans and ieee 8021x defines a portbased network access control protocol which forms the basis for the authentication mechanisms used in vlans but it is also found in wlans it is what the home user sees when the user has to enter a wireless access key ethernet ethernet sometimes simply called lan is a family of protocols used in wired lans described by a set of standards together called ieee 8023 published by the institute of electrical and electronics engineers wireless lan wireless lan also widely known as wlan or wifi is probably the most wellknown member of the ieee 802 protocol family for home users today it is standardized by ieee 80211 and shares many properties with wired ethernet internet protocol suite the internet protocol suite also called tcpip is the foundation of all modern networking it offers connectionless as well as connectionoriented 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 specifications for internet protocol version 4 ipv4 and for ipv6 the next generation of the protocol with a much enlarged addressing capability sonetsdh 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 realtime uncompressed circuitswitched voice encoded in pcm pulsecode modulation format however due to its protocol neutrality and transportoriented features sonetsdh 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 timedivision multiplexing and encodes data into small fixedsized cells this differs from other protocols such as the internet protocol suite or ethernet that use variable sized packets or frames atm has similarity with both circuit and packet switched networking this makes it a good choice for a network that must handle both traditional highthroughput data traffic and realtime lowlatency content such as voice and video atm uses a connectionoriented 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 nextgeneration networks it still plays a role in the last mile which is the connection between an internet service provider and the home user cellular standards there are a number of different digital cellular standards including global system for mobile communications gsm general packet radio service gprs cdmaone cdma2000 evolutiondata optimized evdo enhanced data rates for gsm evolution edge universal mobile telecommunications system umts digital enhanced cordless telecommunications dect digital amps is136tdma and integrated digital enhanced network iden geographic scale computer network typesby spatial scope nanoscale nearfield nfc body ban personal pan nearme nan local lan home han storage san wireless wlan campus can backbone metropolitan man wide wan cloud ian internet interplanetary internet vte a network can be characterized by its physical capacity or its organizational purpose use of the network including user authorization and access rights differ accordingly nanoscale network a nanoscale communication network has key components implemented at the nanoscale including message carriers and leverages physical principles that differ from macroscale communication mechanisms nanoscale communication extends communication to very small sensors and actuators such as those found in biological systems and also tends to operate in environments that would be too harsh for classical communication 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 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 a home school office building or closely positioned group of buildings each computer or device on the network is a node wired lans are most likely based on ethernet technology newer standards such as itut ghn also provide a way to create a wired lan using existing wiring such as coaxial cables telephone lines and power lines the defining characteristics of a lan in contrast to a wide area network wan include higher data transfer rates limited geographic range and lack of reliance on leased lines to provide connectivity current ethernet or other ieee 8023 lan technologies operate at data transfer rates up to 100gbits standardized by ieee in 2010 currently 400gbits ethernet is being developed a lan can be connected to a wan using a router home area network a home area network han is a residential lan 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 mediumsized business environments campus area network a campus area network can is made up of an interconnection of lans within a limited geographical area the networking equipment switches routers and transmission media optical fiber copper plant cat5 cabling etc are almost entirely owned by the campus tenant owner an enterprise university government etc for example a university campus network is likely to link a variety of campus buildings to connect academic colleges or departments the library and student residence halls backbone network a backbone network is part of a computer network infrastructure that provides a path for the exchange of information between different lans or subnetworks a backbone can tie together diverse networks within the same building across different buildings or over a wide area for example a large company might implement a backbone network to connect departments that are located around the world the equipment that ties together the departmental networks constitutes the network backbone when designing a network backbone network performance and network congestion are critical factors to take into account normally the backbone networks capacity is greater than that of the individual networks connected to it another example of a backbone network is the internet backbone which is the set of wide area networks wans and core routers that tie together 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 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 a wan uses a communications channel that combines many types of media such as telephone lines cables and air waves a wan often makes use of 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 enterprise private network an enterprise private network is a network that a single organization builds to interconnect its office locations eg production sites head offices remote offices shops so they can share computer resources virtual private network a virtual private network vpn is an overlay network in which some of the links between nodes are carried by open connections or virtual circuits in some larger network eg 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 besteffort 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 pointtopoint global area network 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 lans organizational scope networks are typically managed by the organizations that own them private enterprise networks may use a combination of intranets and extranets they may also provide network access to the internet which has no single owner and permits virtually unlimited global connectivity intranet an intranet is a set of networks that are under the control of a single administrative entity the intranet uses the ip protocol and ipbased tools such as web browsers and file transfer applications the administrative entity limits use of the intranet to its authorized users most commonly an intranet is the internal lan of an organization a large intranet typically has at least one web server to provide users with organizational information an intranet is also anything behind the router on a local area network extranet an extranet is a network that is also under the administrative control of a single organization but supports a limited connection to a specific external network for example an organization may provide access to some aspects of its intranet to share data with its business partners or customers these other entities are not necessarily trusted from a security standpoint network connection to an extranet is often but not always implemented via wan technology internetwork an internetwork is the connection of multiple computer networks via a common routing technology using routers internet partial map of the internet based on the january 15 2005 data found on opteorg each line is drawn between two nodes representing two ip addresses the length of the lines are indicative of the delay between those two nodes this graph represents less than 30 of the class c networks reachable the internet is the largest example of an internetwork it 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 advanced research projects agency network arpanet developed by 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 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 darknet a darknet is an overlay network typically running on the internet that is only accessible through specialized software a darknet is an anonymizing network where connections are made only between trusted peers sometimes called friends f2f using nonstandard protocols and ports darknets are distinct from other distributed peertopeer networks as sharing is anonymous that is ip addresses are not publicly shared and therefore users can communicate with little fear of governmental or corporate interference routing routing calculates good paths through a network for information to take for example from node 1 to node 6 the best routes are likely to be 1876 or 18106 as this has the thickest routes routing is the process of selecting network paths to carry network traffic routing is performed for many kinds of networks including circuit switching networks and packet switched networks in packet switched networks routing directs packet forwarding the transit of logically addressed network packets from their source toward their ultimate destination through intermediate nodes intermediate nodes are typically network hardware devices such as routers bridges gateways firewalls or switches generalpurpose computers can also forward packets and perform routing though they are not specialized hardware and may suffer from limited performance the routing process usually directs forwarding on the basis of routing tables which maintain a record of the routes to various network destinations thus constructing routing tables which are held in the routers memory is very important for efficient routing there are usually multiple routes that can be taken and to choose between them different elements can be considered to decide which routes get installed into the routing table such as sorted by priority prefixlength where longer subnet masks are preferred independent if it is within a routing protocol or over different routing protocol metric where a lower metriccost is preferred only valid within one and the same routing protocol administrative distance where a lower distance is preferred only valid between different routing protocols most routing algorithms use only one network path at a time multipath routing techniques enable the use of multiple alternative paths routing in a more narrow sense of the term is often contrasted with bridging in its assumption that network addresses are structured and that similar addresses imply proximity within the network structured addresses allow a single routing table entry to represent the route to a group of devices in large networks structured addressing routing in the narrow sense outperforms unstructured addressing bridging routing has become the dominant form of addressing on the internet bridging is still widely used within localized environments network service network services are applications hosted by servers on a computer network to provide some functionality for members or users of the network or to help the network itself to operate the world wide web email services are usually based on a service protocol that defines the format and sequencing of messages between clients and servers of that network service network performance quality of service depending on the installation requirements network performance is usually measured by the quality of service of a telecommunications product the parameters that affect this typically can include throughput jitter bit error rate and latency the following list gives examples of network performance measures for a circuitswitched network and one type of packetswitched network viz atm circuitswitched 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 other types of performance measures can include the level of noise and echo 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 there are many 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 for example state transition diagrams are often used to model queuing performance in a circuitswitched network the network planner uses these diagrams to analyze how the network performs in each state ensuring that the network is optimally designed network congestion network congestion occurs when a link or node is carrying so much data that its quality of service deteriorates typical effects include queueing delay packet loss or the blocking of new connections a consequence of these latter two is that incremental increases in offered load lead either only to small increase in network throughput or to an actual reduction in network throughput network protocols that use aggressive retransmissions to compensate for packet loss tend to keep systems in a state of network congestioneven after the initial load is reduced to a level that would not normally induce network congestion thus networks using these protocols can exhibit two stable states under the same level of load the stable state with low throughput is known as congestive collapse modern networks use congestion control congestion avoidance and traffic control techniques to try to avoid congestion collapse these include exponential backoff in protocols such as 80211s csmaca and the original ethernet window reduction in tcp and fair queueing in devices such as routers another method to avoid the negative effects of network congestion is implementing priority schemes so that some packets are transmitted with higher priority than others priority schemes do not solve network congestion by themselves but they help to alleviate the effects of congestion for some services an example of this is 8021p a third method to avoid network congestion is the explicit allocation of network resources to specific flows one example of this is the use of contentionfree transmission opportunities cftxops in the itut ghn standard which provides highspeed up to 1gbits local area networking over existing home wires power lines phone lines and coaxial cables for the internet rfc2914 addresses the subject of congestion control in detail network resilience network resilience is the ability to provide and maintain an acceptable level of service in the face of faults and challenges to normal operation security computer security network security network security consists of provisions and policies adopted by the network administrator to prevent and monitor unauthorized access misuse modification or denial of the computer network and its networkaccessible 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 is used on a variety of computer networks both public and private to secure daily transactions and communications among businesses government agencies and individuals network surveillance network surveillance is the monitoring of data being transferred over computer networks such as the internet the monitoring is often done surreptitiously and may be done by or at the behest of governments by corporations criminal organizations or individuals it may or may not be legal and may or may not require authorization from a court or other independent agency computer and network surveillance programs are widespread today and almost all internet traffic is or could potentially be monitored for clues to illegal activity surveillance is very useful to governments and law enforcement to maintain social control recognize and monitor threats and preventinvestigate criminal activity with the advent of programs such as the total information awareness program technologies such as high speed surveillance computers and biometrics software and laws such as the communications assistance for law enforcement act governments now possess an unprecedented ability to monitor the activities of citizens however many civil rights and privacy groupssuch as reporters without borders the electronic frontier foundation and the american civil liberties unionhave expressed concern that increasing surveillance of citizens may lead to a mass surveillance society with limited political and personal freedoms fears such as this have led to numerous lawsuits such as hepting v att end to end encryption endtoend encryption e2ee is a digital communications paradigm of uninterrupted protection of data traveling between two communicating parties it involves the originating party encrypting data so only the intended recipient can decrypt it with no dependency on third parties endtoend encryption prevents intermediaries such as internet providers or application service providers from discovering or tampering with communications endtoend encryption generally protects both confidentiality and integrity examples of endtoend encryption include https for web traffic pgp for email otr for instant messaging zrtp for telephony and tetra for radio typical serverbased communications systems do not include endtoend encryption these systems can only guarantee protection of communications between clients and servers not between the communicating parties themselves examples of none2ee systems are google talk yahoo messenger facebook and dropbox some such systems for example lavabit and secretink have even described themselves as offering endtoend encryption when they do not some systems that normally offer endtoend encryption have turned out to contain a back door that subverts negotiation of the encryption key between the communicating parties for example skype or hushmail the endtoend encryption paradigm does not directly address risks at the communications endpoints themselves such as the technical exploitation of clients poor quality random number generators or key escrow e2ee also does not address traffic analysis which relates to things such as the identities of the end points and the times and quantities of messages that are sent ssltls the introduction and rapid growth of ecommerce on the world wide web in the mid1990s made it obvious that some form of authentication and encryption was needed netscape took the first shot at a new standard at the time the dominant web browser was netscape navigator netscape created a standard called secure socket layer ssl ssl requires a server with a certificate when a client requests access to an sslsecured server the server sends a copy of the certificate to the client the ssl client checks this certificate all web browsers come with an exhaustive list of ca root certificates preloaded and if the certificate checks out the server is authenticated and the client negotiates a symmetrickey cipher for use in the session the session is now in a very secure encrypted tunnel between the ssl server and the ssl client 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 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 peertopeer technologies network administrators can see networks from both physical and logical perspectives the physical perspective involves geographic locations physical cabling and the network elements eg routers bridges and application layer gateways that interconnect via the transmission media logical networks called in the tcpip architecture subnets map onto one or more transmission 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 vlan technology both users and administrators are aware to varying extents of the trust and scope characteristics of a network again using tcpip architectural terminology an intranet is a community of interest under private administration usually by an enterprise and is only accessible by authorized users eg employees 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 humanreadable 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 businesstobusiness b2b businesstoconsumer b2c and consumertoconsumer c2c communications when money or sensitive information is exchanged the communications are apt to be protected 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 journals and newsletters open computer science open access journal comparison of network diagram software cyberspace history of the internet network simulation network planning and design network traffic control minimumpairs protocol this article incorporatespublic domain material from the general services administration document federal standard 1037c shelly gary et al discovering computers 2003 edition 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 topdown approach featuring the internet pearson education 2005 william stallings computer networking with internet protocols and technology pearson education 2004 important publications in computer networks network communication architecture and protocols osi network architecture 7 layers model dimitri bertsekas and robert gallager data networks prentice hall 1992 networking at curlie ieee ethernet manufacturer information a computer networking acronym guide vtetelecommunicationshistory beacon broadcasting cable protection system cable tv communications satellite computer network drums electrical telegraph fax heliographs hydraulic telegraph internet mass media mobile phone optical telecommunication optical telegraphy pager photophone prepay mobile phone radio radiotelephone satellite communications semaphore smartphone smoke signals telecommunications history telautograph telegraphy teleprinter teletype telephone the telephone cases television timeline of communication technology undersea telegraph line videoconferencing videophone videotelephony whistled languagepioneers edwin howard armstrong john logie baird paul baran alexander graham bell tim bernerslee jagadish chandra bose vint cerf claude chappe donald davies lee de forest philo farnsworth reginald fessenden elisha gray erna schneider hoover charles k kao hedy lamarr innocenzo manzetti guglielmo marconi antonio meucci radia perlman alexander stepanovich popov johann philipp reis nikola tesla camille tissot alfred vail charles wheatstone vladimir k zworykin transmissionmedia coaxial cable fiberoptic communication optical fiber freespace optical communication molecular communication radio waves transmission line network topologyand switching links nodes terminal node network switchingcircuit packet telephone exchange multiplexing spacedivision frequencydivision timedivision polarizationdivision orbital angularmomentum codedivision networks arpanet bitnet cellular network computer cyclades ethernet fidonet internet isdn lan mobile ngn npl network public switched telephone radio telecommunications equipment television telex wan wireless world wide web category list portal commons wikiproject vteoperating systemsgeneral advocacy comparison forensic engineering history hobbyist development list timeline usage share kernelarchitectures exokernel hybrid microkernel monolithic rump kernel unikernel components device driver loadable kernel module microkernel user space process managementconcepts context switch interrupt ipc process process control block realtime thread timesharing schedulingalgorithms computer multitasking fixedpriority preemptive multilevel feedback queue preemptive roundrobin shortest job next memory management andresource protection bus error general protection fault memory protection paging protection ring segmentation fault virtual memory storage access andfile systems boot loader defragmentation device file file attribute inode journal partition virtual file system virtual tape library list amigaos android beos bsd chrome os cpm dos genode gnu haiku illumos includeos ios linux macintosh classic mac os macos minix morphos musicsp nemesis nextstep nos openvms orvyl os2 osv pick qnx reactos risc os rstse rsx11 rt11 solaris tops10tops20 tpf unix visi on vmcms vs9 webos windows xinu zos miscellaneous concepts api computer network hal live cd live usb os shell cli gui nui tui vui zui pxe vtemajor fields of computer sciencenote this template roughly follows the 2012 acm computing classification systemhardware printed circuit board peripheral integrated circuit very large scale integration systems on chip socs energy consumption green computing electronic design automation hardware acceleration computer systemsorganization computer architecture embedded system realtime computing dependability networks network architecture network protocol network components network scheduler network performance evaluation network service software organization interpreter middleware virtual machine operating system software quality software notationsand tools programming paradigm programming language compiler domainspecific language modeling language software framework integrated development environment software configuration management software library software repository software development software development process requirements analysis software design software construction software deployment software maintenance programming team opensource model theory of computation model of computation formal language automata theory computational complexity theory logic semantics algorithms algorithm design analysis of algorithms algorithmic efficiency randomized algorithm computational geometry mathematicsof computing discrete mathematics probability statistics mathematical software information theory mathematical analysis numerical analysis informationsystems database management system information storage systems enterprise information system social information systems geographic information system decision support system process control system multimedia information system data mining digital library computing platform digital marketing world wide web information retrieval security cryptography formal methods security services intrusion detection system hardware security network security information security application security humancomputerinteraction interaction design social computing ubiquitous computing visualization accessibility concurrency concurrent computing parallel computing distributed computing multithreading multiprocessing artificialintelligence natural language processing knowledge representation and reasoning computer vision automated planning and scheduling search methodology control method philosophy of artificial intelligence distributed artificial intelligence machine learning supervised learning unsupervised learning reinforcement learning multitask learning crossvalidation graphics animation rendering image manipulation graphics processing unit mixed reality virtual reality image compression solid modeling appliedcomputing ecommerce enterprise software computational mathematics computational physics computational chemistry computational biology computational social science computational engineering computational healthcare digital art electronic publishing cyberwarfare electronic voting video games word processing operations research educational technology document management book category portal wikiproject commons authority control gnd 40700859 ndl 00865620 nkc ph115867 