Routers are network devices that forward packets of data between different networks. A router between an organisation's LAN and Janet will not have a direct connection to every other router on the Internet. It is however possible to set up that router to forward packets to their destinations by the most efficient path. The router does this by referring to its routing tables, which list all the possible paths that data can take to get from source to destination IP address. Each router on the path repeats this process until the data reaches its final destination.

Routers permit each connected network to maintain its independent identity and IP address and can also facilitate the implementation of security procedures. It is possible for example to control access to a network from the outside world by using packet filtering.