Routers may also be used to connect two or more logical groups of computer devices known as subnets, each with a different sub-network address. The subnet addresses recorded in the router do not necessarily map directly to the physical interface connections.[3]
A router has two stages of operation called planes:[4]
Control plane: A router maintains a routing table that lists which route should be used to forward a data packet, and through which physical interface connection. It does this using internal pre-configured directives, called static routes, or by learning routes using a dynamic routing protocol. Static and dynamic routes are stored in the Routing Information Base (RIB). The control-plane logic then strips the RIB from non essential directives and builds a Forwarding Information Base (FIB) to be used by the forwarding-plane.
Forwarding plane: The router forwards data packets between incoming and outgoing interface connections. It routes them to the correct network type using information that the packet header contains. It uses data recorded in the routing table control plane.
Routers may provide connectivity within enterprises, between enterprises and the Internet, or between internet service providers' (ISPs) networks. The largest routers (such as the Cisco CRS-1 or Juniper T1600) interconnect the various ISPs, or may be used in large enterprise networks.[5] Smaller routers usually provide connectivity for typical home and office networks. Other networking solutions may be provided by a backbone Wireless Distribution System (WDS), which avoids the costs of introducing networking cables into buildings.All sizes of routers may be found inside enterprises.[6] The most powerful routers are usually found in ISPs, academic and research facilities. Large businesses may also need more powerful routers to cope with ever increasing demands of intranet data traffic. A three-layer model is in common use, not all of which need be present in smaller networks.[7]