Internet Core Protocols: The Definitive Guide

The ARP Cache

When the requesting system gets an ARP response, it will store the hardware and IP address pair of the requested device into a local cache. The next time that system needs to send data, it will check the local cache, and if an entry is found it will go ahead and use it, eliminating the need to broadcast another request.

Likewise, the system that responded to the ARP broadcast will store the hardware and IP addresses of the system that sent the original broadcast. If it did not do so, it would have to issue an ARP broadcast to find out where to send the ARP response.

Since all of the systems on the network will see the ARP broadcast, they could go ahead and capture the IP and hardware addresses of the sender, storing this information in their own caches. However, doing this might cause existing entries in the cache to be flushed (an especially problematic issue with systems that have small caches), so only those systems who already have the sender’s IP address in their cache should update their entries. Hosts that do not already have the device in their cache should ignore the broadcast.