ARP
Networking
The protocol that finds a neighbour's MAC address from its IP address, by broadcasting the question to the whole local network. It only ever works on your own link — the manual for the table it fills says it binds addresses "for hosts sharing the same link" — which is why a destination off the network goes through the gateway, a neighbour. ip neigh show prints the cache: REACHABLE for a binding confirmed recently, STALE for one that is valid but unused, FAILED or INCOMPLETE when nobody answered. Nothing behind a router can appear in it.
Also written: neighbour table, ip neigh