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Definition: router

A network device that forwards data packets from one network to another. Based on the address of the destination network in the incoming packet and an internal routing table, the router determines which port (line) to send out the packet (ports typically connect to Ethernet cables). Routers require packets formatted in a routable protocol, the global standard being TCP/IP, or simply "IP."

In the Home
In the home or small office, a "wireless router" is commonly used to manage Internet traffic. It is a combination device that houses a router, network switch and Wi-Fi in one box (for details, see wireless router).

In a Company
Enterprise routers are dedicated to packet forwarding only and can cost from 10 to more than a hundred times as much as a wireless router. They take factors such as traffic load, external line costs and congestion into consideration to determine which port to forward to. Measured in millions of packets per second (see PPS), large routers can handle enormous amounts of traffic.

Routers in the Core
Within the 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 wide area networks (WANs). 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. See Ethernet, SONET, edge router and collapsed backbone.

Routers, Switches and Access Points
In a local network (LAN), routers connect to Ethernet switches. The router forwards packets between networks, and the switch forwards packets between machines within a network. Wi-Fi is provided by separate base stations that plug into the switches (see access point).

Routers work at the network layer (layer 3) of the protocol, whereas switches work at the data link layer (layer 2), also known as the "MAC layer." See TCP/IP and data link protocol.

Specialized Machines or Regular PCs
Routers are specialized computer-based devices optimized for communications; however, router software can be added to a server. For example, NAT32 (www.nat32.com) is Windows software that lets a PC function as a router to the Internet.

The major router vendors for enterprises and service providers are Cisco, Alcatel-Lucent, Huawei, Juniper and ZTE.

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 (network layer 3), 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.)