What is the purpose of Neighbor Advertisement (NA) in NDP?

The purpose of Neighbor Advertisement (NA) within the Neighbor Discovery Protocol (NDP) is to facilitate various functionalities in IPv6 networks, such as resolving link-layer addresses, discovering neighbors, and maintaining reachability information about neighboring nodes. NA messages are crucial in the process of address resolution, similar to ARP in IPv4 networks.

Neighbor Advertisements are specifically defined by ICMPv6 as Type 136. When a node on the network receives a Neighbor Solicitation (NS) message, it responds with a Neighbor Advertisement (NA) if it is the target of the NS or if it has information to provide back to the querying node. This mechanism allows nodes in an IPv6 network to efficiently communicate their presence and link-layer addresses, ensuring effective connectivity and communication.

The other options pertain to different types within the ICMPv6 specification: Type 133 is designated for Router Solicitation, Type 134 is for Router Advertisement, and Type 135 is for Neighbor Solicitation. Understanding these various types further clarifies how NDP works as a whole, emphasizing the unique role that Neighbor Advertisements play in maintaining the operational efficiency of an IPv6 network.