ARP and IPv6 Neighbor Discovery on a local network

On a local network, knowing a neighbour’s IP address is not enough: the device also needs the link-layer address. IPv4 uses ARP for this job, while IPv6 uses ICMPv6-based Neighbor Discovery.

How devices find neighbours on the local network

ARP resolves an IPv4 neighbour to a link-layer address on the local link. IPv6 does not use ARP: Neighbor Discovery uses ICMPv6 for neighbour resolution, router discovery and reachability functions.

How to inspect the neighbour table

Inspect the neighbour table while testing a host on the same LAN. An incomplete or failed entry points to local neighbour resolution rather than DNS or Internet routing. On IPv6, also check that ICMPv6 Neighbor Discovery is not being filtered. To check which subnet an address belongs to, use the 2ip IP calculator.

ARP and Neighbor Discovery are different mechanisms

Technical reference: RFC 826 and RFC 4861.