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A router is a networking device that forwards data packets between computer networks. Routers perform the "traffic directing" functions on the Internet. A data packet is typically forwarded from one router to another through the networks that constitute the internetwork until it reaches its destination node.〔(【引用サイトリンク】title=Overview Of Key Routing Protocol Concepts: Architectures, Protocol Types, Algorithms and Metrics )〕 A router is connected to two or more data lines from different networks (as opposed to a network switch, which connects data lines from one single network). When a data packet comes in on one of the lines, the router reads the address information in the packet to determine its ultimate destination. Then, using information in its routing table or routing policy, it directs the packet to the next network on its journey. This creates an overlay internetwork. The most familiar type of routers are home and small office routers that simply pass data, such as web pages, email, IM, and videos between the home computers and the Internet. An example of a router would be the owner's cable or DSL router, which connects to the Internet through an ISP. More sophisticated routers, such as enterprise routers, connect large business or ISP networks up to the powerful core routers that forward data at high speed along the optical fiber lines of the Internet backbone. Though routers are typically dedicated hardware devices, use of software-based routers has grown increasingly common. == Applications == When multiple routers are used in interconnected networks, the routers exchange information about destination addresses using a dynamic routing protocol. Each router builds up a table listing the preferred routes between any two systems on the interconnected networks.〔(【引用サイトリンク】url=http://www.ciscopress.com/articles/article.asp?p=2180210&seqNum=4 )〕 A router has interfaces for different physical types of network connections, such as copper cables, fibre optic, or wireless transmission. It also contains firmware for different networking communications protocol standards. Each network interface uses this specialized computer software to enable data packets to be forwarded from one protocol transmission system to another. Routers may also be used to connect two or more logical groups of computer devices known as subnets, each with a different sub-network address. The subnet addresses recorded in the router do not necessarily map directly to the physical interface connections.〔(Requirements for IPv4 Routers ),RFC 1812, F. Baker, June 1995〕 A router has two stages of operation called planes:〔(Requirements for Separation of IP Control and Forwarding ),RFC 3654, H. Khosravi & T. Anderson, November 2003〕 * Control plane: A router maintains a routing table that lists which route should be used to forward a data packet, and through which physical interface connection. It does this using internal pre-configured directives, called static routes, or by learning routes using a dynamic routing protocol. Static and dynamic routes are stored in the Routing Information Base (RIB). The control-plane logic then strips the RIB from non essential directives and builds a Forwarding Information Base (FIB) to be used by the forwarding-plane. * Forwarding plane: The router forwards data packets between incoming and outgoing interface connections. It routes them to the correct network type using information that the packet header contains. It uses data recorded in the routing table control plane. Routers may provide connectivity within enterprises, between enterprises and the Internet, or between internet service providers' (ISPs) networks. The largest routers (such as the Cisco CRS-1 or Juniper T1600) interconnect the various ISPs, or may be used in large enterprise networks.〔(【引用サイトリンク】title=Setting uo Netflow on Cisco Routers )〕 Smaller routers usually provide connectivity for typical home and office networks. Other networking solutions may be provided by a backbone Wireless Distribution System (WDS), which avoids the costs of introducing networking cables into buildings. All sizes of routers may be found inside enterprises.〔(【引用サイトリンク】title=Windows Home Server: Router Setup )〕 The most powerful routers are usually found in ISPs, academic and research facilities. Large businesses may also need more powerful routers to cope with ever increasing demands of intranet data traffic. A three-layer model is in common use, not all of which need be present in smaller networks. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Router (computing)」の詳細全文を読む スポンサード リンク
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