To reach this website, your computer sent some packets across the Internet. If we’re curious what that path was, we can run a tool to generate a traceroute — a rough list of every server your packets touched to reach their destination. To build this website (source code on GitHub), I wrote my own traceroute program called ktr (also open source) that can stream results in real time while concurrently looking up interesting information about each hop.

How does ktr work? Let’s start with a simplified explanation of Internet routing.

Starting with the source device, each computer that handles a packet has to choose the best device to forward it to — I will explain how these routing decisions are made in a bit. Assuming everything works correctly, the packet will eventually reach a router that knows how to send it directly to its destination.