Anycast is communication between a single sender and several receivers topologically nearest in a group. The term exists in contradistinction to multicast, communication between a single sender and a group of selected receivers, and unicast, communication between one sender and one receiver in a network.

Anycast is used in IPv6 as a method of updating routing tables. One host initiates an update of a router table for a group of hosts, sending the data to the nearest host. IPv6 can determine which gateway host is closest and sends the packets to that host as though it were a unicast communication. That host then sends the message on to its nearest router until all the routing tables in that group are updated.

IPv6 has unicast, multicast, and anycast. Broadcast has disappeared as a term, but is considered one form of multicast. In IPv6 there are three addresses associated with a network device: unicast address, multicast address and anycast address. IPv6 does not distinguish between unicast and anycast addresses during routing. A node receiving an anycast packet needs to put its own unicast address as sender address in the replying packet header. Anycast routing uses Address Resolution Protocol (ARP) or Link-Level multicast for advertisements and listening. When forwarding packets, it is similar to unicasting in the network and the last-hop routing is similar to that in multicasting.

Anycast   Illustration

Anycast   Illustration

Related Terms: Multicast, Broadcast, Unicast, ARP, IPv6

Reference Links: http://www.javvin.com/protocol/rfc1546.txt: Host Anycasting Service