Anycast

Anycast assigns the same IP address to several nodes in different locations, and the routing system delivers each packet to whichever of them is closest in routing terms. It sits beside the other delivery models: unicast (one to one), broadcast (one to all on a segment) and multicast (one to a subscribed group). The IPv6 addressing architecture (RFC 4291) defines anycast addresses; on the internet the same effect is commonly achieved for IPv4 by announcing one prefix from several sites with BGP.

Anycast has availability value. The DNS root servers, many large DNS services and content distribution networks use it, so that users reach a nearby instance and a failed site stops attracting traffic once its route is withdrawn. It also spreads a distributed denial of service flood across many sites, because each attacking source reaches its own nearest node rather than one shared target. The dependency is routing: a false or withdrawn route announcement changes which node receives the traffic.

Exam relevance: questions in this area tend to turn on matching a delivery model to its description. “One to the nearest of many” is anycast. Candidates are expected not to confuse it with multicast, where every member of the group receives a copy, and to recognise anycast as an availability measure for services such as DNS.