Anycast – the basics

Each server on a network (like the Internet) is usually assigned an address and each address is usually assigned to a single server.  Anycast is when you assign the same address to multiple servers and use routing configurations to make sure traffic is routed to the correct server.  On private networks where there is no overlap this is pretty easy to manage (just don’t route the Anycast addresses out of the closed network).  On the public Internet things are somewhat more complicated since routes change regularly so a given machine could end up talking to different servers at different points in time as routing changes happen on the Internet (congested links, outages, and for hundreds of other reasons).

The routing behavior on a network as large as the Internet means Anycast is not a good fit for stateful long-lived connections but stateless protocols or protocols that recover well can still work.  Luckily for the web, the two foundational protocols for web traffic are largely stateless (DNS and HTTP).

via Performance Matters: Anycast and what it means for web performance