Router Definition

A router is an electronic device and/or software that connects at least two networks and forwards packets among them according to the information in the packet headers and routing tables. Routers are fundamental to the operation of the Internet and other complex networks (such as enterprise-wide networks).

A network consists of two or more computers, and typically other devices as well (such as printers and external hard drives), that are linked together so that they can communicate with each other and thereby share files and the devices. Examples of the networks connected by a router can be two LANs (local area networks) or WANs (wide area networks) or a LAN and its ISP's (Internet service provider's) network.

A packet is the fundamental unit of information transport in all modern computer networks, and increasingly in other communications networks as well. A packet header is the portion of a data packet that precedes the body (i.e., a portion of the message being transmitted) and which contains source and destination IP addresses as well as control and timing information required for successful transmission.

Routing, which is the moving of packets across networks using the most appropriate paths, occurs at the network layer of the OSI seven-layer model. This layer, which is the third from the bottom, is also responsible for addressing messages and translating logical addresses (i.e., IP addresses) into physical addresses (i.e., MAC addresses).

Routers route messages transmitted only by a routable protocol such as IP (Internet protocol) or IPX (internetwork packet exchange). Some routers support only a single protocol; multiprotocol routers support more than one protocol. Messages sent using non-routable protocols, such as NetBIOS, cannot be routed, but they can be transferred between LANs via bridges.

Routers use protocols such as Internet control message protocol (ICMP) to communicate with each other in order to update their routing tables and configure the most efficient routes between any two hosts (i.e., computers on the network).

Routing functions were initially performed by general purpose computers, but now they are typically performed by dedicated computers that have been optimized for this purpose. Most Unix-like operating systems include all necessary software to perform routing.

Routers can connect networks using different media and architectures. They do not care about the type of data they handle, and they thus perform very little filtering of data, except for broadcasts.

An edge router is a router that routes data between one or more LANs and the Internet. A core router is a router that transmits data between other routers.

A brouter is a single devices that serves as both a router and a bridge. It will route some packets based on network layer information and forward other packets based on data link layer (i.e., the layer at which bridges operate) information.

Created October 17, 2005.
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