ARP (Address Resolution Protocol)

ARP (Address Resolution Protocol) lets a host discover the MAC address of another device on its subnet. It broadcasts "who has this IP?" and the owner replies with its MAC address, which the asker caches.

Why it matters

Ethernet frames need a destination MAC, but applications only know IP addresses. ARP bridges the two on every local hop, including the hop to your default gateway. Because ARP has no authentication, attackers on a LAN can spoof replies (ARP poisoning) to intercept traffic. IPv6 uses Neighbor Discovery instead of ARP.

Example

$ ip neigh
192.168.1.1 dev eth0 lladdr 3c:22:fb:1a:90:7e REACHABLE
192.168.1.40 dev eth0 lladdr 00:1b:44:11:3a:b7 STALE

Each line is a cached IP-to-MAC mapping. Entries expire after a short time and are refreshed on demand.

Common mistakes

• Expecting ARP to work across routers. It is limited to one broadcast domain.
• Ignoring duplicate IP addresses, which cause two MACs to answer one ARP request and intermittent connectivity.
• Leaving a flat network without protections such as dynamic ARP inspection where untrusted users connect.

Related

• MAC address
• Broadcast
• Subnet
• Network switch

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