An IPv6 address assigned to multiple nodes sends a message to only one node. What is this called?

The scenario described involves an IPv6 address assigned to multiple nodes but only sending messages to one specific node. This is known as an anycast address. Anycast addresses allow for communication between a source and the nearest node in a group of nodes that share the same address. When a packet is sent to an anycast address, it is delivered to the closest interface identified by that address, as determined by the routing protocol.

In the context of networking, this means that even though multiple nodes can share an anycast address, the messaging behavior is such that the message will only be received by one of those nodes, specifically the one which is closest in terms of network topology.

Understanding this concept is critical, as it differentiates anycast from other types of addresses. For example, a multicast address would allow messages to be sent to multiple nodes simultaneously, while a unicast address sends messages to a single unique recipient. Broadcast addresses, which are more prevalent in IPv4, send messages to all nodes on the network, but are not applicable in IPv6 the same way. Thus, the unique characteristic of anycast is its ability to direct messages to just one node from a group, facilitating efficient routing based on proximity.