A router , pronounced /ˈraʊtər/ in the United States, Canada, and Australia, and /ˈruːtər/ in the UK and Ireland (to differentiate it from the tool used to rout wood), is an electronic device used to connect two or more computers or other electronic devices to each other, and usually to the Internet, by wire or radio signals. This allows several computers to communicate with each other and to the Internet at the same time. If wires are used, each computer is connected by its own wire to the router. Modern wired-only routers designed for the home or small business typically have one "input" port (to the Internet) and four "output" ports, one or more of which can be connected to other computers. A typical modern home wireless router , in addition to having four wired ports, also allows several devices to connect with it wirelessly. Most modern personal computers are built with a wired port (almost always an Ethernet type), which allows them to connect to a router with the addition of just a cable (typically a Category 5e type). To connect with a wireless router, a device must have an adapter. This is sometimes, but not always, included with the computer at manufacture. Some electronic games, including handheld electronic games, have an adapter built-in, or one can be added later.
More technically, a router is a networking device whose software and hardware are usually tailored to the tasks of routing and forwarding information. Routers connect two or more logical subnets, which do not necessarily map one-to-one to the physical interfaces of the router. The term "layer 3 switching" is often used interchangeably with routing, but switch is a general term without a rigorous technical definition. In marketing usage, a switch is generally optimized for Ethernet LAN interfaces and may not have other physical interface types. In comparison, the network hub (predecessor of the "switch" or "switching hub") does not do any routing, instead every packet it receives on one network line gets forwarded to all the other network lines.
Routers operate in two different planes:
- Control plane, in which the router learns the outgoing interface that is most appropriate for forwarding specific packets to specific destinations,
- Forwarding plane, which is responsible for the actual process of sending a packet received on a logical interface to an outbound logical interface.
Forwarding plane (a.k.a. data plane)
Main article: Forwarding planeFor the pure Internet Protocol (IP) forwarding function, router design tries to minimize the state information kept on individual packets. Once a packet is forwarded, the router should no longer retain statistical information about it. It is the sending and receiving endpoints that keeps information about such things as errored or missing packets.
Forwarding decisions can involve decisions at layers other than the IP internetwork layer or OSI layer 3. Again, the marketing term switch can be applied to devices that have these capabilities. A function that forwards based on data link layer, or OSI layer 2, information, is properly called a bridge. Marketing literature may call it a layer 2 switch, but a switch has no precise definition.
Among the most important forwarding decisions is deciding what to do when congestion occurs, i.e., packets arrive at the router at a rate higher than the router can process. Three policies commonly used in the Internet are Tail drop, Random early detection, and Weighted random early detection. Tail drop is the simplest and most easily implemented; the router simply drops packets once the length of the queue exceeds the size of the buffers in the router. Random early detection (RED) probabilistically drops datagrams early when the queue exceeds a configured size. Weighted random early detection requires a weighted average queue size to exceed the configured size, so that short bursts will not trigger random drops.
A router uses a routing table to decide where the packet should be sent so if the router cant find the preferred address then it will look down the routing table and decide which is the next best address to send it to.
Types of routers
Routers may provide connectivity inside enterprises, between enterprises and the Internet, and inside Internet Service Providers (ISPs). The largest routers (for example the Cisco CRS-1 or Juniper T1600) interconnect ISPs, are used inside ISPs, or may be used in very large enterprise networks. The smallest routers provide connectivity for small and home offices.
Routers for Internet connectivity and internal use
Routers intended for ISP and major enterprise connectivity will almost invariably exchange routing information with the Border Gateway Protocol (BGP). RFC 4098 defines several types of BGP-speaking routers:
- Edge Router: Placed at the edge of an ISP network, it speaks external BGP (eBGP) to a BGP speaker in another provider or large enterprise Autonomous System(AS).
- Subscriber Edge Router: Located at the edge of the subscriber's network, it speaks eBGP to its provider's AS(s). It belongs to an end user (enterprise) organization.
- Inter-provider Border Router: Interconnecting ISPs, this is a BGP speaking router that maintains BGP sessions with other BGP speaking routers in other providers' ASes.
- Core router: A router that resides within the middle or backbone of the LAN network rather than at its periphery.
Routers are also used for port forwarding for private servers.
Small Office Home Office (SOHO) connectivity
Main article: Residential gatewayResidential gateways (often called routers) are frequently used in homes to connect to a broadband service, such as IP over cable or DSL. Such a router may also include an internal DSL modem. Residential gateways and SOHO routers typically provide network address translation and port address translation in addition to routing. Instead of directly presenting the IP addresses of local computers to the remote network, such a residential gateway makes multiple local computers appear to be a single computer. SOHO routers may also support Virtual Private Network tunnel functionality to provide connectivity to an enterprise network..
Enterprise routers
All sizes of routers may be found inside enterprises. The most powerful routers tend to be found in ISPs and academic & research facilities. Large businesses may also need powerful routers.
A three-layer model is in common use, not all of which need be present in smaller networks.
Access
Access routers, including SOHO, are located at customer sites such as branch offices that do not need hierarchical routing of their own. Typically, they are optimized for low cost.
Distribution
Distribution routers aggregate traffic from multiple access routers, either at the same site, or to collect the data streams from multiple sites to a major enterprise location. Distribution routers often are responsible for enforcing quality of service across a WAN, so they may have considerable memory, multiple WAN interfaces, and substantial processing intelligence.
They may also provide connectivity to groups of servers or to external networks. In the latter application, the router's functionality must be carefully considered as part of the overall security architecture. Separate from the router may be a Firewalled or VPN concentrator, or the router may include these and other security functions.
When an enterprise is primarily on one campus, there may not be a distinct distribution tier, other than perhaps off-campus access. In such cases, the access routers, connected to LANs, interconnect via core routers.
Core
In enterprises, a core router may provide a "collapsed backbone" interconnecting the distribution tier routers from multiple buildings of a campus, or large enterprise locations. They tend to be optimized for high bandwidth.
When an enterprise is widely distributed with no central location(s), the function of core routing may be subsumed by the WAN service to which the enterprise subscribes, and the distribution routers become the highest tier.
History
The very first device that had fundamentally the same functionality as a router does today, i.e a packet switch, was the Interface Message Processor (IMP); IMPs were the devices that made up the ARPANET, the first packet switching network. The idea for a router (although they were called "gateways" at the time
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