Definition of:Router


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 smaller routers for less intensive applications. (Image courtesy of Cisco Systems, Inc.)

Definition of:router

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

In the home or small office, a wireless router is commonly used to direct traffic to and from the Internet. It combines a router, network switch and Wi-Fi in one box (see wireless router). However, in a large company, routers are also used to separate local networks (LANs) into subnetworks (subnets) to balance traffic within workgroups and to filter traffic for security purposes and policy management. Large-scale enterprise routers take traffic load, congestion, line costs and other factors into consideration to determine which port to forward to.

Routers in the Core
Within the large enterprise, routers serve as an internet (lower case "i") backbone that connects all internal networks, in which case they are typically connected via Ethernet twisted pair metal wires. Within the global Internet (upper case "I"), routers do all the packet switching between the backbones and are typically connected via SONET fiber links. See Ethernet, SONET and collapsed backbone.

Routers and Switches
In a local area network (LAN), routers work in conjunction with Ethernet switches. The router forwards packets from network to network, and the switch forwards packets from machine to machine. 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
Most routers are specialized computer-based devices optimized for communications; however, router functions can also be implemented by adding software to a server. For example, NAT32 is Windows software that lets a PC function as a router to the Internet. The major router vendors of equipment 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), seeOSI 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.