A router is a network device that forwards traffic depending on the destination address of that traffic. Basic routing has just a few steps:

a) The router receives a packet of data on one of its interfaces.

b) The router examines the destination IP of this packet.

c) The router then looks up the destination network of this IP in its routing table.

d) The router forwards that out through the interface that's closest to the remote network.

These steps are repeated as often as needed until the traffic reaches its destination. For each next hop and each destination network, the router will have to keep track of how far away that destination currently is. Interface, the router also has to know which of its interfaces it should use for traffic matching the destination network out of.

Routers use what is known as routing protocols. These are special protocols the routers use to speak to each other in order to share what information they might have. Routing protocols fall into two main categories, interior gateway protocols, and exterior gateway protocols.