The Neighbor Discovery Protocol (NDP) is a protocol in the Internet Protocol Suite used with Internet Protocol Version 6 (IPv6). It operates in the Link Layer of the Internet model (RFC 1122) and is responsible for address autoconfiguration of nodes, discovery of other nodes on the link, determining the Link Layer addresses of other nodes, duplicate address detection, finding available routers, address prefix discovery, and maintaining reachability information about the paths to other active neighbor nodes (RFC 4861
).

In its function for address resolution for IPv6, NDP performs functions similar to the Address Resolution Protocol (ARP) and Internet Control Message Protocol (ICMP) Router Discovery and Router Redirect protocols for IPv4. However, it provides many improvements over its IPv4 counterparts (RFC 4861, section 3.1). For example, it includes Neighbor Unreachability Detection (NUD), thus improving robustness of packet delivery in the presence of failing routers or links, or mobile nodes.