The Neighbor Discovery (ND) protocol is an important part of the Internet Protocol suite used with IPv6. NDP in IPv6 is a replacement of Address Resolution Protocol (ARP) and ICMP Router Discovery (RD) in IPv4. NDP uses ICMPv6 packets to implement functions including RD, duplicate address detection (DAD), address resolution, neighbor unreachability detection (NUD), and redirection.
If two hosts need to communicate on a local area network (LAN), the network-layer address (IPv6 address) of the receiver must be available to the sender. In addition, IPv6 data packets must be encapsulated into frames before they are sent over a physical network. Therefore, the sender must know the physical address (MAC address) of the receiver, and the mapping between the IPv6 address and the physical address must be available to ensure transmission of data packets.
NDP allows mapping between network-layer IPv6 address and link-layer MAC addresses to ensure communication on an Ethernet.