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Definition: router

A device that forwards data from one network to another. Based on an internal routing table and the address of the destination network in the incoming packet, the router determines whether to send the packet out or keep it within the network. The traffic traversing a router is formatted in a routable protocol, the global standard being TCP/IP, or simply IP (see TCP/IP and routable protocol).

At Home
In the home or small office, a "wireless router" or "wireless gateway" is used to manage Internet traffic. The wireless router contains a built-in Ethernet switch and Wi-Fi access point (see wireless router), and the wireless gateway also includes a modem (see cable/DSL gateway). Both router and gateway have a single port to the Internet.

In a Company
In the enterprise, routers in the local network (LAN) are dedicated to packet forwarding and connect to stand-alone modems and Wi-Fi access points. Although routers do have built-in Ethernet ports, they are often connected to additional Ethernet switches to reach more computers and printers. See Ethernet switch and access point.

Routers in the Core
Within a large enterprise, routers separate local area networks (LANs) into subnetworks (subnets) to balance traffic within workgroups and to filter traffic for security purposes and policy management. They also forward packets between the company's LANs and external networks (WANs), as well as to the Internet. Factors such as traffic load, external line costs and congestion may be taken into consideration to determine which port to forward to. See LAN and WAN.

Within the Internet, very large-scale routers do all the packet switching between the national and regional backbones and are typically connected via optical fibers. Measured in millions of packets per second (see PPS), large routers handle enormous amounts of traffic. See packet switching, Ethernet, SONET, edge router and collapsed backbone.

Specialized Machines or Regular PCs
Routers are dedicated communications devices, and the major router vendors for enterprises and service providers are Cisco, Alcatel-Lucent, Huawei, Juniper and ZTE. Linksys, NETGEAR and D-Link make popular consumer routers. However, a Windows PC can be turned into a router with software such as NAT32 (www.nat32.com).

Router Terminology
Routers used to be called "gateways," which is why the term "default gateway" means the router in your network (see default gateway). For more details on the routable protocol layer, see OSI model and TCP/IP abc's. See layer 3 switch, route server, router cluster and routing protocol.

Route Forwarding
Routing tables hold the data for making forwarding decisions. Although this is a simple example, routing tables become very complex. Static routing uses fixed tables, but dynamic routing uses routing protocols that let routers exchange data with each other.

Cisco Routers
For years, Cisco has been the leading router vendor, and these high-end, carrier-grade 7600 models process up to 30 million packets per second (pps). Cisco also makes faster routers as well as smaller routers for less intensive applications. (Image courtesy of Cisco Systems, Inc.)