Our final topic is routing, or how packets are directed through the maze of networks in order to arrive at their destination host. Routing is trivial on LANs because the topologies are either very simple or very regular. In an Ethernet, every transmitted message is heard by every attached host, called broadcasting, so routing is unnecessary. Token rings always transmit packets to their one downwind neighbor, so routing is trivially simple. Star topologies also have trivial routing. All messages that a host sends are sent to the hub, which then forwards them to their destination. Only in meshes is routing tricky.

The original ARPANET was a mesh with a number of minicomputers called IMPs (Interface Message Processors) acting as the routers. Hosts attached to these IMPs, so hosts didn't have to worry about routing -- they sent everything to their attached IMP. However, the IMPs used a sophisticated algorithm for determining the best route for packets.