One address, a thousand doors

Badges collected live from a home connection.

Before you start. How a parcel finds any house on earth gave you the routing table: a list of rows, each one a range of addresses and a next hop, and a router that only ever asks “which neighbour do I hand this to,” never “where does this end up.” TCP vs UDP, and the 3-way handshake gave you the other half: a DNS question is one packet out and one packet back, with no ongoing conversation, so nothing is lost if the reply next time comes from somewhere new.

The idea in 30 seconds

The same address can live in more than one building at once, and ordinary routing sends each visitor to the nearest one, with nobody in the middle deciding a thing.
Routers tell each other which addresses they can reach, in a language called BGP: “I can reach this address, through me,” passed neighbour to neighbour.
A router keeps whichever claim travelled the shortest trail, which usually, not always, means nearest.
A building going down is just a withdrawn claim. The next-best building the neighbours already knew about takes over, with nothing to switch on.
The trick suits a one-packet question like DNS best, because there is no ongoing conversation to break if the answer starts coming from somewhere new.
The same idea explains why a 2002 flood barely dented the internet, and why a 2007 flood only hurt the two doors that hadn’t spread out yet.

Read the story · Skip to the numbers