Routing Packets Between Networks

Routing is how a packet finds its way from one network to another: each router on the path looks up the destination in a table and sends the packet one hop closer. Knowing how that choice is made explains why traffic takes the path it does, why a cloud network sends a request where it does, and what the common diagnostic tools can and cannot prove.

The topic opens on routing tables and the rule that picks one route when several match, with an example on a single address. It then shows how ping and traceroute use a hop counter to see the path, compares the protocols that build routes inside one organisation and between organisations, and explains anycast, where one address lives in many places. A second example applies the same rule to a cloud route table, and a gotcha shows what a successful ping does not prove.

How a router delivers the packet to the next device is the data link layer's job, covered earlier.

By the end you can predict which route a router chooses, read a traceroute, and say what a ping does and does not show.