neighbour table
also: ARP table, neighbour cache, MAC table
A kernel data structure that maps IP addresses to hardware (MAC) addresses on a local network segment, primarily managed through the ARP protocol for IPv4 and NDP for IPv6.
The neighbour table is part of the Linux kernel's networking stack and acts as a cache of IP-to-MAC address mappings for devices on the same physical network. When a host needs to send a packet to another host on the local network, it must know that host's MAC address; the neighbour table stores these mappings to avoid repeated ARP (Address Resolution Protocol) requests.
You can view and manage the neighbour table using the ip neighbour command (or ip n for short). For example, ip neighbour show displays all known neighbours, including their IP addresses, MAC addresses, and reachability state (REACHABLE, STALE, FAILED, etc.).
The neighbour table operates in different states: REACHABLE (recently confirmed), STALE (valid but unconfirmed), FAILED (unreachable), and others. The kernel automatically ages entries and triggers ARP requests when needed to keep mappings current and detect link failures on the local segment.