A router has multiple interfaces and receives data packets through them. It evaluates the network addresses of the incoming packets and decides which interface to forward the packet to. It uses its local routing table for decision-making. This can be statically configured or calculated via dynamic routing protocols such as OSPF or BGP.
Routers are optimised to suit a particular purpose, depending on their application. So-called backbone routers are high-performance routers of the carrier class, which route and forward packets with rapid speeds of several gigabits per second. They are housed in data centres, and may be as large as several 19-inch cabinets.
For interfacing with networks of other providers, Internet service providers may use border routers or edge routers, which mainly use the routing protocol BGP. This routing protocol allows for the optimum the exchange of routes. Most of these routers also support the prioritisation of traffic via Quality of Service.
For connecting to the Internet, access routers are used, which allow devices in a local area network to access Internet via DSL, cable, wireless or ISDN.
Many Internet routers have additional functions for telephony. They often have full telephone systems integrated within the devices. Analogue or digital telephones may be connected to them, as well as cordless DECT phones or Voice-over-IP phones, depending on the type. While a normal Internet connection provides sufficient access for VoIP telephony, routers with telephone systems for standard telephony need interfaces for access to the analogue or digital (ISDN) telephone network.