<P> 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 sender's 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). </P> <P> 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 . </P> <P> 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 . </P> <P> 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 . </P>

What data fields does the host look up to determine whether it needs to reply to an arp request