an answer for . Computer 2 responds with an ARP response message containing its MAC and IP addresses. As part of fielding the request, Computer 2 may insert an entry for Computer 1 into its ARP table for future use. Computer 1 receives and caches the response information in its ARP table and can now send the packet. ARP probe An ARP probe in IPv4 is an ARP request constructed with the SHA of the probing host, an SPA of all 0s, a THA of all 0s, and a TPA set to the IPv4 address being probed for. If some host on the network regards the IPv4 address (in the TPA) as its own, it will reply to the probe (via the SHA of the probing host) thus informing the probing host of the address conflict. If instead there is no host which regards the IPv4 address as its own, then there will be no reply. When several such probes have been sent, with slight delays, and none receive replies, it can reasonably be expected that no conflict exists. As the original probe packet contains neither a valid SHA/SPA nor a valid THA/TPA pair, there is no risk of any host using the packet to update its cache with problematic data. Before beginning to use an IPv4 address (whether received from manual configuration, DHCP, or some other means), a host implementing this specification must test to see if the address is already in use, by broadcasting ARP probe packets. ARP announcements 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 changes. Such an announcement, also called a gratuitous ARP (GARP) message, is usually broadcast as an ARP request containing the SPA in the target field (TPA=SPA), with THA set to zero. An alternative way is to broadcast an ARP reply with the sender's SHA and SPA duplicated in the target fields (TPA=SPA, THA=SHA). The ARP request and ARP reply announcements are both standards-based methods, but the ARP request method is preferred. Some devices may be configured for the use of either of these two types of announcements. 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 in the