IPv6 Anycast Address
Anycast is basically the same on IPv4 and IPv6 so this part below refers to both.
As the name says it’s an address that can exist more than once anywhere in the network. If we look public IP space that’s available on the Internet, anycast IPv6 address can exist on multiple places all over the Internet. This kind of address is basically enabling us to have servers and services physically closer to us as they would be if the unicast address was used. It this way we are able to have, for example, a server with one anycast IP address somewhere in US and other server with same service and same IP address somewhere is Europe. If I am in Europe, closest server with that IP address will handle my request. Without to much additional technology solutions my service will automatically be resolved to server who is closer to me and it will probably also improve service security and speed. All that is called load balancing and can be accomplished by different networking solutions and technology designs but anycast addressing is basically the simplest method possible to enable this kind of “geo” localization for a service.
How anycast works?
As said before anycast addresses are called anycast because one address can be assigned to multiple interfaces inside the same network. Packets that are going to anycast IP destination address will be caught by nearest device. Today’s anycast IP addresses are used on some special routers and the most important thing that runs them is Global Internet’s DNS root servers service. Google also rely on anycast for all his different solutions and apps like gmail, search and so on.
If you imagine how DNS works you can see why anycast would be used on root DNS servers. You can then have one copy of the same DNS server on each continent. BGP will by himself bring your DNS query to server near you and in that way save you some delay time and bandwidth usage and thus some time.
In IPv6 world, what changes?
IPv6 had from the development phase the intention to support anycast just like described from RFC 1546. (RFC 1546 mentioned below in history section). IPv6 anycast has no special prefix and IPv6 anycast addresses are basically normal global unicast addresses. Each IPv6 configured interface on some device needs to be configured with one anycast address.
There is a big chance that anycast interfaces have no defined region, in that case every anycast entry would need to be propagated throughout the whole Internet. That would probably not scale well so support for that kind of global anycast addresses will be more or less impossible to handle.
If there are regions defined, inside the region devices with same anycast address will only need a separate entry in the routing table.