ARP may also be used as a simple announcement protocol. This is useful for updating other hosts' mappings of a hardware address when the sender's IP address or MAC address has changed. Such an announcement, also called a gratuitous ARP message, is usually broadcast as an ARP request containing the senders protocol address (SPA) in the target field (TPA=SPA), with the target hardware address (THA) set to zero. An alternative way is to broadcast an ARP reply with the sender's hardware and protocol addresses (SHA and SPA) duplicated in the target fields (TPA=SPA, THA=SHA).
The gratuitous ARP request message and the gratuitous ARP reply messages are standards-based methods, but the "ARP Request" is preferred. Some devices may be configured for the use of either of these two types of GARP.
An ARP announcement is not intended to solicit a reply; instead it updates any cached entries in the ARP tables of other hosts that receive the packet. The operation code may indicate a request or a reply because the ARP standard specifies that the opcode is only processed after the ARP table has been updated from the address fields.
Many operating systems perform gratuitous ARP during startup. That helps to resolve problems which would otherwise occur if, for example, a network card was recently changed (changing the IP-address-to-MAC-address mapping) and other hosts still have the old mapping in their ARP caches.
Gratuitous ARP is also used by some interface drivers to provide load balancing for incoming traffic. In a team of network cards, it is used to announce a different MAC address within the team that should receive incoming packets.
ARP announcements can be used to defend link-local IP addresses in the Zeroconf protocol (RFC 3927), and for IP address takeover within high-availability clusters.

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