Neighbor Discovery in IPv6 is a fundamental process that enables devices within the same network segment to identify and communicate with their neighboring devices, just like the Address Resolution Protocol (ARP) does in IPv4 networks. It is critical to the operation of a network, supporting functions such as address resolution, router discovery, and duplicate address detection.
As you can see in the image above, the Neighbor Discovery process works by exchanging ICMPv6 messages between devices. An ICMPv6 Neighbor Solicitation message is sent by a device when it wants to communicate with a neighbor or resolve an IP address to a MAC address. The target device replies with a Neighbor Advertisement message that includes its MAC address. Router Discovery uses Router Advertisement and Router Solicitation messages to identify routers and configure default routes.