Neighbour Unreachability Detection works in the following manner. When an IPv6 host has a packet to send, it checks the Neighbour Cache to determine the link layer address of the next hop node (either an on-link neighbour or a router). The Neighbour cache also has an associated state with each neighbour entry. A neighbour state of REACHABLE indicates that the neighbour is considered reachable.

In IPv6 a host considers a neighbour reachable if it has recently received confirmation that packets sent to the neighbour have been received. This is achieved in two ways: the receipt of a neighbour advertisement from the neighbour in response to a neighbour solicitation sent by the host or a hint from upper layer protocols. The IPv6 stack utilises the acknowledgements of upper layer protocols to register the fact that a packet has recently been received from a given destination address and so is considered reachable.

The IPv6 host will send a neighbour solicitation in the event that the neighbour cache entry not being set as REACHABLE when there is a packet to send.