Which ICMPv6 type is Neighbor Solicitation?

• A. 128 - Echo Request
• B. 129 - Echo Reply
• C. 135 - Neighbor Solicitation
• D. 136 - Neighbor Advertisement

Answer: (C)

Neighbor discovery in IPv6 uses ICMPv6 messages to find and verify neighbors. The message used to discover a neighbor's link-layer address and to verify reachability is the Neighbor Solicitation. It’s sent to the neighbor’s solicited-node multicast address to request its link-layer address or to confirm it can be reached. The neighbor answers with a Neighbor Advertisement containing its link-layer address. This is why the Neighbor Solicitation type is the correct choice—it’s the specific message designed for querying a neighbor's address and reachability within the Neighbor Discovery protocol. Echo Request and Echo Reply serve ping-like reachability tests, while Neighbor Advertisement is the response to a solicitation or an unsolicited address advertisement.