A host may occasionally issue an ARP Request with its own IP address as the target address. These ARP
Requests, known as gratuitous ARPs, have two uses:
A gratuitous ARP may be used for duplicate address checks. A device that issues an ARP Request
with its own IP address as the target and receives an ARP Reply from another device will know
that the address is a duplicate.
A gratuitous ARP may be used to advertise a new data link identifier. This use takes advantage of
the fact that when a device receives an ARP Request for an IP address that is already in its ARP
cache, the cache will be updated with the sender's new hardware address.
Many IP implementations do not use gratuitous ARP, but you should be aware of its existence.