What Is Anycast?

How it works

An operator announces the same prefix, say 1.1.1.0/24, via BGP from data centres around the world. Routers pick the shortest path, so a user in Berlin reaches the Frankfurt site and a user in Tokyo the Tokyo site, with the same address.

Why use it

• Speed: answers come from nearby.
• Resilience: if one site goes down, its routes are withdrawn and traffic flows to the next nearest.
• DDoS absorption: attack traffic spreads across all sites instead of hitting one.

Compared with other delivery types

Unicast is one address, one place; multicast is one address, a group of receivers; anycast is one address, many places, one receiver per user.

Why it confuses IP location

Geolocation databases give one location per IP, but an anycast IP is everywhere. That's why 1.1.1.1 might be listed as being in one country while you're talking to a server in your own city.

$ dig +short CHAOS TXT id.server @1.1.1.1

Cloudflare's resolver answers this query with a short code for the site that served you, based on the nearest airport's code. Ask from another country and you'll get a different site, for the same address.

traceroute 1.1.1.1 from different places ends at a nearby hop with only a few milliseconds of latency, even though the address is the same everywhere.

LightAtom's Map places addresses where their network is registered, and reminds you that CDN and anycast addresses may really be served from much closer.