ARP Cache Life
Since the source checks its local ARP cache prior to sending packets on the network, the ARP cache must contain current information.
If the ARP cache contains stale information, data packets could be routed to the wrong host.
To ensure that old ARP table entries do not become a serious problem, network devices place a time-to-live (TTL) on ARP entries.
By using TTLs, an operating system ensures that an ARP cache never is outdated for more than the specified time.
The process of removing ARP entries from an ARP table is known as aging out the ARP table.