To send an IP packet over a local link, a host often needs the link-layer address of the next hop.

In IPv4, ARP maps an on-link IPv4 address to a MAC address. A host can broadcast a question such as “who has 192.0.2.1?” and cache the reply associating that IP address with a MAC address.

IPv6 uses Neighbor Discovery instead of ARP. Neighbor Discovery uses IPv6 control messages to resolve neighboring link-layer addresses and also participates in discovering local routers.

The crucial point is that resolution concerns the next local hop, not necessarily the final IP destination. If the destination is outside the host's local prefix, the host resolves the link-layer address of its default router and sends the frame there.

Cached mappings reduce repeated discovery traffic but can become stale when interfaces or topology change.