An IPv6 anycast address is an address that is assigned to more than one interface (typically belonging to different nodes), with the property that a datagram sent to an anycast address is routed to the "nearest" interface having that address, according to the routing protocols' measure of distance.
All anycast addresses can be forwarded using standard methods. It is enough that the computer, which joins the anycast address, announces this fact to any router. This router already arranges the distribution of the information.
Anycast addresses are allocated from the unicast address space, using any of the defined unicast address formats. Thus, anycast addresses are syntactically indistinguishable from unicast addresses. When a unicast address is assigned to more than one interface, thus turning it into an anycast address, the nodes to which the address is assigned must be explicitly configured to know that it is an anycast address.
For any assigned anycast address, there is a longest prefix P of that address that identifies the topological region in which all interfaces belonging to that anycast address reside. Within the region identified by P, the anycast address must be maintained as a separate entry in the routing system (commonly referred to as a "host route"). Outside the region identified by P, the anycast address may be aggregated into the routing entry for prefix P.
Note that in the worst case, the prefix P of an anycast set may be the null prefix, i.e., the members of the set may have no topological locality. In that case, the anycast address must be maintained as a separate routing entry throughout the entire Internet, which presents a severe scaling limit on how many such "global" anycast sets may be supported.
Another restriction of use anycast addresses is a dynamic routing and routing policy of tier 1 ISP. Dynamic routing causes problems for protocols, such as TCP protocol, and routing policy of tier 1 ISP refuse too long prefixes and thus the anycast addresses with too long prefix P. In addition, the different parts of the Internet core network are managed by different organizations whose routing policy we cannot control.
Therefore, it is expected that support for global anycast sets may be unavailable or very restricted.
One expected use of anycast addresses is to identify the set of routers belonging to an organization providing Internet service. Such addresses could be used as intermediate addresses in an IPv6 Routing header, to cause a packet to be delivered via a particular service provider or sequence of service providers.
Some other possible uses are to identify the set of routers attached to a particular subnet, or the set of routers providing entry into a particular routing domain.
Currently, it is expected that global anycast sets will mainly be used for DNS servers. On the other hand, anycast addresses can be a more effective way of addressing for the smaller sites that are managed by a single ISP.