Routing tables

A routing table is a list of networks and where to send packets for each one. Every router has one, and so does your computer. Each row has a destination network (a prefix like 10.1.0.0/16), a next hop (the router to hand the packet to) or an interface to send it out of, and a cost.

Destination      Next hop      Interface
10.1.0.0/16      10.0.0.2      eth1
10.1.5.0/24      10.0.0.3      eth1
192.168.1.0/24   (direct)      eth0
0.0.0.0/0        203.0.113.1   eth2

When a packet arrives, the router finds every row whose prefix contains the destination address, then picks the most specific one: the longest prefix match. A packet for 10.1.5.9 matches both 10.1.0.0/16 and 10.1.5.0/24, so it goes to 10.0.0.3, because /24 is more specific than /16. A packet for 10.1.9.9 only matches the /16, so it goes to 10.0.0.2.

0.0.0.0/0 matches every address, with a prefix length of zero, so it only wins when nothing else does. It's the default route: "if you don't know, send it this way". Your computer's default route points at its default gateway; a home router's default route points at the internet provider.

Networks a router is plugged into directly are added for it. Others are added either by hand (static routes) or learned from other routers using a routing protocol, which is the next lesson. If no row matches at all and there's no default route, the packet is dropped and an error goes back to the sender.