Routers are used to connect separate networks to form an internetwork. When two or more independent networks are connected, but they continue to function separately, an internetwork is created. The best known internetwork is Internet.

Routers can connect complex networks with multiple paths between network segments. Each network segment (subnet or subnetwork), is assigned a network address. Each node is assigned an address. Using combination of these addresses, the router can route a packet from the source to a destination address on the network. Routers operate at the Network layer of the OSI model. In a network with multiple paths, a router can determine the best path for a packet to take to reach its destination. When a router receives a packet whose destination address is unknown, it discards the packet. Corrupted packets and broadcasts are also discarded.

Routing table is a reference table that include network information and the next router in line for a particular path. By using static routing, the router always sends the packet along the same path. If a router uses dynamic routing, it can make a decision on which path to send the packet down based on the cost of the packet travelling a particular path.

Router can connect networks using same protocols but different network architectures such as Ethernet and token ring or FDDI (Fiber Distributed Data Interface) networks.