Anycast: one IP address from multiple network locations

Several sites originate the same prefix and routers select among the available routes. “Nearest” usually means preferable according to routing policy; it does not necessarily mean the shortest geographic distance.

How one IP can be served from multiple sites

DNS services and CDNs commonly use this design. It can shorten paths and provide resilience, but it cannot guarantee identical latency for every user.

How to observe Anycast in practice

After a BGP policy or topology change, the same IP can lead to another site. Consequently, IP location information may describe the network or registered location rather than the particular edge server that answered.

Compare paths from different networks using the traceroute guide. Different routes to the same Anycast address are expected. For an additional check, use 2ip WHOIS/RDAP lookup.

What not to infer from Anycast

Anycast does not replace application load balancing, state synchronisation or origin security. A configuration fault can also affect several sites at once. Operational guidance for DNS Anycast is documented in RFC 4786.