ARP Overview

The Address Resolution Protocol (ARP) maps IPv4 network addresses to MAC hardware addresses.

When forwarding traffic, a device needs to know the destination MAC address because each IP packet is encapsulated in an Ethernet frame. The MAC address is needed for the packet's final destination and for a next hop toward the destination.

A device first searches its ARP cache and a match for the IP address supplies the corresponding MAC address. Otherwise, the device broadcasts an ARP request. Network devices receive the requests, and the host with a matching IP address sends an ARP reply that includes its MAC address.

After the device receives a matching ARP reply, the following events occur.

The packet is sent toward its destination.
The IP address/MAC address pair is added to the ARP cache as a dynamic ARP entry.

Note

Neighbor Discovery is the technology by which a device gets the MAC address for IPv6. For more information, see IPv6 Neighbor Discovery.