A network device that forwards packets from one network to another. Based on internal routing tables, routers read each incoming packet and decide how to forward it. The destination address in the packets determines which line (interface) outgoing packets are directed to. In large-scale enterprise routers, the current traffic load, congestion, line costs and other factors determine which line to forward to.
At the Edge and In Between
Most routers in the world sit in homes and small offices and do nothing more than direct Web, e-mail and other Internet transactions from the local network to the cable or DSL modem, which is connected to the ISP and Internet. Sitting at the edge of the network, they often contain a built-in firewall for security, and the firewall serves all users in the network without requiring that the personal firewall in each computer be turned on and configured. See firewall and personal firewall.
However, in the larger company, routers are also used to separate local area networks (LANs) into subnetworks (subnets) in order to balance traffic within workgroups and to filter traffic for security purposes and policy management.
Routers in the Core
Within a 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. Within the global Internet (upper case "I"), routers do all the packet switching between the backbones and are typically connected via T3, ATM or SONET links. See collapsed backbone.
Routable Protocols
Routers route messages transmitted only by a routable protocol such as IP or IPX. Multiprotocol routers support more than one protocol; for example, IP "and" IPX. Messages in non-routable protocols, such as NetBIOS and LAT, cannot be routed, but they can be transferred from LAN to LAN via a bridge.
Because routers have to inspect the network address in the packet, they do more processing and add more overhead than a bridge or switch. Routers work at the network layer (layer 3) of the protocol, whereas bridges and switches work at the data link layer (layer 2), also known as the "MAC layer." See OSI model.
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 software from Microsoft that enables a PC to function as a router to the Internet for machines on the network. The major router vendors are Cisco Systems and Nortel Networks.
Router Terminology
Routers used to be called "gateways," which is why the term "default gateway" means the router in your network (see default gateway). In older Novell terminology, routers were also called "network-layer bridges." 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 smaller routers for less intensive
applications. (Image courtesy of Cisco Systems, Inc.)
