How a parcel finds any house on earth

A real package tracked live from a laptop to example.com: 11 handoffs, 15 ms door to door.

Before you start. Nothing here leans on another lesson. One picture runs all the way through: a router is a depot that only knows its own street, a packet is a parcel, and a counter stamped on the parcel, the handoff counter, stops a lost one from circling forever (it gets its own section below). This lesson is about how a depot decides which way to hand a parcel on, when it has never seen a map of the whole postal system.

The idea in 30 seconds

Nobody has a map of the whole internet, not YouTube, not Pakistan Telecom, not anyone. Every depot only knows its own short list of rows.
A row in that list is a prefix: a range of addresses, and which way to hand parcels addressed inside it.
Every router holds a list of rows, each saying “addresses in this range, send this way”. When two rows cover the same address, the narrower one wins (longest-prefix match), on your laptop and on every router on earth.
Every network is its own autonomous system, and BGP is the conversation where autonomous systems tell each other which addresses they own.
Nothing checks whether an autonomous system is telling the truth. In 2008, Pakistan Telecom claimed a narrow slice of YouTube’s addresses, and the ordinary trust between depots sent YouTube’s traffic worldwide into a hole in Pakistan.
The handoff counter on every parcel is what stops a wrongly routed one from circling the internet forever while a mess like that gets sorted out.

Read the story · Skip to the numbers