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A computer network is an interconnection of a group of computers. Networks may be classified by what is called the network layer at which they operate according to basic reference models considered as standards in the industry such as the four-layer Internet Protocol Suite model. While the seven-layer Open Systems Interconnection (OSI) reference model is better known in academia, the majority of networks use the Internet Protocol Suite (IP) as their network model.
Computer networks may be classified according to the scale: Personal area network (PAN), Local Area Network (LAN), Campus Area Network (CAN), Metropolitan area network (MAN), or Wide area network (WAN). As Ethernet increasingly is the standard interface to networks, these distinctions are more important to the network administrator than the end user. Network administrators may have to tune the network, based on delay that derives from distance, to achieve the desired Quality of Service (QoS). The primary difference in the networks is the size.
Controller Area Networks are a special niche, as in control of a vehicle's engine, a boat's electronics, or a set of factory robots.
By connection methodEdit
Computer networks can also be classified according to the hardware technology that is used to connect the individual devices in the network such as Optical fiber, Ethernet, Wireless LAN, HomePNA, or Power line communication.
Ethernets use physical wiring to connect devices. Often, they employ the use of hubs, switches, bridges, and routers.
Wireless LAN technology is built to connect devices without wiring. These devices use a radio frequency to connect.
By functional relationship (Network Architectures)Edit
Computer networks may be classified according to the functional relationships which exist between the elements of the network, e.g., Active Networking, Client-server and Peer-to-peer (workgroup) architectures.
By network topologyEdit
Template:Main Computer networks may be classified according to the network topology upon which the network is based, such as Bus network, Star network, Ring network, Mesh network, Star-bus network, Tree or Hierarchical topology network, etc.
Network Topology signifies the way in which intelligent devices in the network see their logical relations to one another. The use of the term "logical" here is significant. That is, network topology is independent of the "physical" layout of the network. Even if networked computers are physically placed in a linear arrangement, if they are connected via a hub, the network has a Star topology, rather than a Bus Topology. In this regard the visual and operational characteristics of a network are distinct; the logical network topology is not necessarily the same as the physical layout.
Computer networks may be classified according to the communications protocol that is being used on the network. See the articles on List of network protocol stacks and List of network protocols for more information. For a development of the foundations of protocol design see Srikant 2004  and Meyn 2007 
Types of networks: Edit
Below is a list of the most common types of computer networks in order of scale.
Personal Area Network (PAN)Edit
Template:Main A personal area network (PAN) is a computer network used for communication among computer devices close to one person. Some examples of devices that may be used in a PAN are printers, fax machines, telephones, PDAs or scanners. The reach of a PAN is typically within about 20-30 feet (approximately 4-6 Meters). PANs can be used for communication among the individual devices (intrapersonal communication), or for connecting to a higher level network and the Internet (an uplink).
Personal area networks may be wired with computer buses such as USB and FireWire. A wireless personal area network (WPAN) can also be made possible with network technologies such as IrDA and Bluetooth.
Local Area Network (LAN)Edit
Template:Main A network covering a small geographic area, like a home, office, or building. Current LANs are most likely to be based on Ethernet technology. For example, a library will have a wired or wireless LAN for users to interconnect local devices (e.g., printers and servers) connect to the internet. All of the PCs in the library are connected by category 5 (Cat5) cable, running the IEEE 802.3 protocol through a system of interconnection devices and eventually connect to the internet. The cables to the servers are on Cat 5e enhanced cable, which will support IEEE 802.3 at 1 Gbps.
The staff computers (bright green) can get to the color printer, checkout records, and the academic network and the Internet. All user computers can get to the Internet and the card catalog. Each workgroup can get to its local printer. Note that the printers are not accessible from outside their workgroup.
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 Mbps Ethernet connections to the user device and a Gigabit Ethernet connection to the central router, could be called "layer 3 switches" because they only have Ethernet interfaces and must understand IP. It would be more correct to call them access routers, where the router at the top is a distribution router that connects to the Internet and academic networks' customer access routers.
The staff have a VoIP network that also connects to both the Internet and the academic network. They could have paths to the central library system telephone switch, via the academic network. Since voice must have the highest priority, it is on the pink network. The VoIP protocols used, such as RSVP, are virtual circuits rather than connectionless forwarding paths.
Depending on the circumstance, the computers in the network might be connected using cables and hubs. Other networks might be connected strictly wirelessly. It depends on the number of PCs that you are trying to connect, the physical layout of your workspace, and the various needs of network. Not shown in this diagram, for example, is a wireless workstation used when shelving books.
The defining characteristics of LANs, in contrast to WANs (wide area networks), include their much higher data transfer rates, smaller geographic range, and lack of a need for leased telecommunication lines. Current Ethernet or other IEEE 802.3 LAN technologies operate at speeds up to 10 Gbit/s. This is the data transfer rate. IEEE has projects investigating the standardization of 100 Gbit/s, and possibly 40 Gbit/s. Inverse multiplexing is commonly used to build a faster aggregate from slower physical streams, such as bringing 4 Gbit/s aggregate stream into a computer or network element with four 1 Gbit/s interfaces.
Campus Area Network (CAN)Edit
Template:Main A network that connects two or more LANs but that is limited to a specific and contiguous geographical area such as a college campus, industrial complex, or a military base. A CAN, may be considered a type of MAN (metropolitan area network), but is generally limited to an area that is smaller than a typical MAN.
This term is most often used to discuss the implementation of networks for a contiguous area. For Ethernet based networks in the past, when layer 2 switching (i.e., bridging (networking) was cheaper than routing, campuses were good candidates for layer 2 networks, until they grew to very large size. Today, a campus may use a mixture of routing and bridging. The network elements used, called "campus switches", tend to be optimized to have many Ethernet-family (i.e., IEEE 802.3) interfaces rather than an arbitrary mixture of Ethernet and WAN interfaces.
Metropolitan Area Network (MAN)Edit
Template:Main A Metropolitan Area Network is a network that connects two or more Local Area Networks or Campus Area Networks together but does not extend beyond the boundaries of the immediate town, city, or metropolitan area. Multiple routers, switches & hubs are connected to create a MAN.
Wide Area Network (WAN)Edit
Template:Main A WAN is a data communications network that covers a relatively broad geographic area (i.e. one city to another and one country to another country) and that often uses transmission facilities provided by common carriers, such as telephone companies. WAN technologies generally function at the lower three layers of the OSI reference model: the physical layer, the data link layer, and the network layer.
Global Area Network (GAN)Edit
Template:Main Global area networks (GAN) specifications are in development by several groups, and there is no common definition. In general, however, a GAN is a model for supporting mobile communications across an arbitrary number of wireless LANs, satellite coverage areas, etc. The key challenge in mobile communications is "handing off" the user communications from one local coverage area to the next. In IEEE Project 802, this involves a succession of terrestrial Wireless local area networks (WLAN) . INMARSAT has defined a satellite-based Broadband Global Area Network (BGAN).
IEEE mobility efforts focus on the data link layer and make assumptions about the media. Mobile IP is a network layer technique, developed by the IETF, which is independent of the media type and can run over different media while still keeping the connection.
Template:Main Two or more networks or network segments connected using devices that operate at layer 3 (the 'network' layer) of the OSI Basic Reference Model, such as a router. Any interconnection among or between public, private, commercial, industrial, or governmental networks may also be defined as an internetwork.
In modern practice, the interconnected networks use the Internet Protocol. There are at least three variants of internetwork, depending on who administers and who participates in them:
- "The" Internet
Intranets and extranets may or may not have connections to the Internet. If connected to the Internet, the intranet or extranet is normally protected from being accessed from the Internet without proper authorization. The Internet itself is not considered to be a part of the intranet or extranet, although the Internet may serve as a portal for access to portions of an extranet.
Template:Main An intranet is a set of interconnected networks, using the Internet Protocol and uses IP-based tools such as web browsers, that is under the control of a single administrative entity. That administrative entity closes the intranet to the rest of the world, and allows only specific users. Most commonly, an intranet is the internal network of a company or other enterprise.
Template:Main An extranet is a network or internetwork that is limited in scope to a single organization or entity but which also has limited connections to the networks of one or more other usually, but not necessarily, trusted organizations or entities (e.g. a company's customers may be given access to some part of its intranet creating in this way an extranet, while at the same time the customers may not be considered 'trusted' from a security standpoint). Technically, an extranet may also be categorized as a CAN, MAN, WAN, or other type of network, although, by definition, an extranet cannot consist of a single LAN; it must have at least one connection with an external network.
Template:Main A specific internetwork , consisting of a worldwide interconnection of governmental, academic, public, and private networks based upon the Advanced Research Projects Agency Network (ARPANET) developed by ARPA of the U.S. Department of Defense – also home to the World Wide Web (WWW) and referred to as the 'Internet' with a capital 'I' to distinguish it from other generic internetworks.
Participants in the Internet, or their service providers, use IP Addresses obtained from address registries that control assignments. Service providers and large enterprises also exchange information on the reachability of their address ranges through the BGP Border Gateway Protocol.
Basic Hardware ComponentsEdit
All networks are made up of basic hardware building blocks to interconnect network nodes, such as Network Interface Cards (NICs), Bridges, Hubs, Switches, and Routers. In addition, some method of connecting these building blocks is required, usually in the form of galvanic cable (most commonly Category 5 cable). Less common are microwave links (as in IEEE 802.11) or optical cable ("optical fiber").
Network Interface CardsEdit
Template:Main A network card, network adapter or NIC (network interface card) is a piece of computer hardware designed to allow computers to communicate over a computer network. It provides physical access to a networking medium and often provides a low-level addressing system through the use of MAC addresses. It allows users to connect to each other either by using cables or wirelessly.
Template:Main A repeater is an electronic device that receives a signal and retransmits it at a higher level or higher power, or onto the other side of an obstruction, so that the signal can cover longer distances without degradation.
Because repeaters work with the actual physical signal, and do not attempt to interpret the data being transmitted, they operate on the Physical layer, the first layer of the OSI model.
Template:Main A hub contains multiple ports. When a packet arrives at one port, it is copied to all the ports of the hub. When the packets are copied, the destination address in the frame does not change to a broadcast address. It does this in a rudimentary way, it simply copies the data to all of the Nodes connected to the hub. 
Template:Main A network bridge connects multiple network segments at the data link layer (layer 2) of the OSI model. Bridges do not promiscuously copy traffic to all ports, as hubs do. but learns which MAC addresses are reachable through specific ports. Once the bridge associates a port and an address, it will send traffic for that address only to that port. Bridges do send broadcasts to all ports except the one on which the broadcast was received.
Bridges learn the association of ports and addresses by examining the source address of frames that it sees on various ports. Once a frame arrives through a port, its source address is stored and the bridge assumes that MAC address is associated with that port. The first time that a previously unknown destination address is seen, the bridge will forward the frame to all ports other than the one on which the frame arrived.
Bridges come in three basic types:
- Local bridges: Directly connect local area networks (LANs)
- Remote bridges: Can be used to create a wide area network (WAN) link between LANs. Remote bridges, where the connecting link is slower than the end networks, largely have been replaced by routers.
- Wireless bridges: Can be used to join LANs or connect remote stations to LANs.
Template:Main Switches are a marketing term that encompasses routers and bridges, as well as devices that may distribute traffic on load or by application content (e.g., a Web URL identifier). Switches may operate at one or more OSI layers, including physical, data link, network, or transport (i.e., end-to-end). A device that operates simultaneously at more than one of these layers is called a multilayer switch.
Overemphasizing the ill-defined term "switch" often leads to confusion when first trying to understand networking. Many experienced network designers and operators recommend starting with the logic of devices dealing with only one protocol level, not all of which are covered by OSI. Multilayer device selection is an advanced topic that may lead to selecting particular implementations, but multilayer switching is simply not a real-world design concept.
Template:Main Routers are the networking device that forward data packets along networks by using headers and forwarding tables to determine the best path to forward the packets. Routers work at the network layer of the TCP/IP model or layer 3 of the OSI model. Routers also provide interconnectivity between like and unlike media (RFC 1812) This is accomplished by examining the Header of a data packet, and making a decision on the next hop to which it should be sent (RFC 1812) They use preconfigured static routes, status of their hardware interfaces, and routing protocols to select the best route between any two subnets. A router is connected to at least two networks, commonly two LANs or WANs or a LAN and its ISP's network. Some DSL and cable modems, for home use, have been integrated with routers to allow multiple home computers to access the Internet.
Building a simple computer network Edit
A simple computer network may be constructed from two computers by adding a network adapter (Network Interface Controller (NIC)) to each computer and then connecting them together with a special cable called a crossover cable. This type of network is useful for transferring information between two computers that are not normally connected to each other by a permanent network connection or for basic home networking applications. Alternatively, a network between two computers can be established without dedicated extra hardware by using a standard connection such as the RS-232 serial port on both computers, connecting them to each other via a special crosslinked null modem cable.
Practical networks generally consist of more than two interconnected computers and generally require special devices in addition to the Network Interface Controller that each computer needs to beuipped with. Examples of some of these special devices are hubs, switches and routers.
Ancillary equipment used by networks Edit
To keep a network operating, to diagnose failures or degradation, and to circumvent problems, networks may have a wide-ranging amount of ancillary equipment.
Providing Electrical Power Edit
Individual network components may have surge protectors - an appliance designed to protect electrical devices from voltage spikes. Surge protectors attempt to regulate the voltage supplied to an electric device by either blocking or shorting to ground voltage above a safe threshold.
Beyond the surge protector, network elements may have uninterruptible power supplies (UPS), which can be anywhere from a line-charged battery to take the element through a brief power dropout, to an extensive network of generators and large battery banks that can protect the network for hours or days of commercial power outages.
A network as simple as two computers linked with a crossover cable has several points at which the network could fail: either network interface, and the cable. Large networks, without careful design, can have many points at which a single failure could disable the network.
When networks are critical the general rule is that they should have no single point of failure. The broad factors that can bring down networks, according to the Software Engineering Institute  at Carnegie-Mellon University:
- Attacks: these include software attacks by various miscreants (e.g., malicious hackers, computer criminals) as well as physical destruction of facilities.
- Failures: these are in no way deliberate, but range from human error in entering commands, bugs in network element executable code, failures of electronic components, and other things that involve deliberate human action or system design.
- Accidents: Ranging from spilling coffee into a network element to a natural disaster or war that destroys a data center, these are largely unpredictable events. Survivability from severe accidents will require physically diverse, redundant facilities. Among the extreme protections against both accidents and attacks are airborne command posts and communications relays, which either are continuously in the air, or take off on warning. In like manner, systems of communications satellites may have standby spares in space, which can be activated and brought into the constellation.
Dealing with Power FailuresEdit
One obvious form of failure is the loss of electrical power. Depending on the criticality and budget of the network, protection from power failures can range from simple filters against excessive voltage spikes, to consumer-grade Uninterruptible Power Supplies(UPS) that can protect against loss of commercial power for a few minutes, to independent generators with large battery banks. Critical installations may switch from commercial to internal power in the event of a brownout,where the voltage level is below the normal minimum level specified for the system. Systems supplied with three-phase electric power also suffer brownouts if one or more phases are absent, at reduced voltage, or incorrectly phased. Such malfunctions are particularly damaging to electric motors. Some brownouts, called voltage reductions, are made intentionally to prevent a full power outage.
Some network elements operate in a manner to protect themselves and shut down gracefully in the event of a loss of power. These might include noncritical application and network management servers, but not true network elements such as routers. UPS may provide a signal called the "Power-Good" signal. Its purpose is to tell the computer all is well with the power supply and that the computer can continue to operate normally. If the Power-Good signal is not present, the computer shuts down. The Power-Good signal prevents the computer from attempting to operate on improper voltages and damaging itself
To help standardize approaches to power failures, the Advanced Configuration and Power Interface (ACPI) specification is an open industry standard first released in December 1996 developed by HP, Intel, Microsoft, Phoenix and Toshiba that defines common interfaces for hardware recognition, motherboard and device configuration and power management.
Monitoring and Diagnostic Equipment Edit
Networks, depending on their criticality and the skill set available among the operators, may have a variety of temporarily or permanently connected performance measurement and diagnostic equipment. Routers and bridges intended more for the enterprise or ISP market than home use, for example, usually record the amount of traffic and errors experienced on their interfaces.
Diagnostic equipment, to isolate failures, may be nothing more complicated than a spare piece of equipment. If the problem disappears when the spare is manually replaced, the problem has been diagnosed. More sophisticated and expensive installations will have spare elements that can automatically replace a failed unit. Failures can be made transparent to user computers with techniques such as the Virtual Router Redundancy Protocol (VRRP), as specified in RFC 3768.
- Active networking
- Computer networking
- Computer networking device
- Expander graph
- History of the Internet
- Institute of Electrical and Electronic Engineers
- International Organization for Standardization
- International Telecommunications Union - Telecommunication Standards Sector
- Network diagram
- Network topology
- Node (networking)
- Scale-free network
- Wireless network
- Bluetooth Networks
- Network tomography
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