Neighbor Discovery is a process whereby host computers and routers can determine the addresses for neighbors known to reside on shared links. Part of this discovery process is accomplished using ICMP messages called Neighbor Solicitations and Neighbor Advertisements. Neighbor Solicitation messages are sent to discover the link-layer address of a neighbor or to verify that a neighbor is still reachable via a cached link-layer address. A Neighbor Advertisement that contains address information is sent in response to a solicitation or to announce an address change.