anycast
also: anycast address, anycast routing
A network addressing scheme where a single IP address is assigned to multiple servers, and traffic is routed to the nearest or best-performing instance. Clients send packets to the anycast address, but only one server receives each packet.
Anycast enables multiple servers in different geographic locations to share the same IP address. When a client sends a packet to that address, routing protocols automatically deliver it to the topologically nearest or most optimal server based on BGP (Border Gateway Protocol) metrics.
Unlike unicast (one-to-one) or multicast (one-to-many), anycast is one-to-one-of-many: each packet reaches only one server, but the client doesn't know or control which one. This makes it ideal for distributed services.
Common uses include DNS root nameservers (all 13 root servers share anycast addresses like 198.41.0.4), content delivery networks (CDNs), and load balancing across global data centers. For example, querying a root nameserver from Tokyo might reach a local instance, while a query from London reaches a different instance of the same anycast address.