Routing

In short: Routing is forwarding data packets across several networks to their destination. Routers do this.

In more detail: A router reads the destination IP address of a packet, looks into its routing table and sends the packet on to the next router (next hop). So it travels step by step until it arrives.

In Depth

• Routing table: it holds destination networks, the next router and a metric (cost). If several entries match, the most specific one wins (longest prefix, see subnet mask).
• Default gateway: the router to which a device sends everything not in its own network.
• Static or dynamic: static routes are entered by hand. Dynamic protocols (RIP, OSPF, BGP) exchange information and adapt to failures. BGP connects the big networks of the internet.
• TTL: every packet has a lifetime that drops at each router. This prevents endless loops.
• Seeing the path: tracert shows all routers on the way.
• Distinction: a switch forwards inside one network by MAC addresses, a router between networks by IP addresses.

See also: router, addressing types, ping