Editing Address Resolution Protocol (section)

==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,{{Ref RFC|5944|rsection=4.6}} but the ''ARP request'' method is preferred.{{Ref RFC|5227|rsection=3|quote=Why Are ARP Announcements Performed Using ARP Request Packets and Not ARP Reply Packets?}} Some devices may be configured for the use of either of these two types of announcements.<ref>{{cite web
 | url = http://support.citrix.com/article/CTX112701
 | title = FAQ: The Firewall Does not Update the Address Resolution Protocol Table
 | publisher = [[Citrix]]
 | date = 2015-01-16
 | quote =  [...] garpReply enabled [...] generates ARP packets that [...] are of OPCODE type REPLY, rather than REQUEST.
}}</ref>

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 announcement may be either request or reply; the ARP standard specifies that the opcode is only processed after the ARP table has been updated from the address fields.<ref>{{cite web|url=http://www1.ietf.org/mail-archive/web/dhcwg/current/msg03797.html|title=Gratuitous ARP in DHCP vs. IPv4 ACD Draft|archive-url=https://web.archive.org/web/20071012093401/http://www1.ietf.org/mail-archive/web/dhcwg/current/msg03797.html |archive-date=October 12, 2007 }}</ref>{{Ref RFC|5944|rsection=4.6}}{{Ref RFC|2131|rsection=4.4.1}}

Many operating systems issue an ARP announcement during startup. This helps to resolve problems that 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.

ARP announcements are also used by some network interfaces to provide load balancing for incoming traffic. In a [[NIC teaming|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 in the [[Zeroconf]] protocol to allow automatic assignment of a [[link-local address]] to an interface where no other IP address configuration is available. The announcements are used to ensure an address chosen by a host is not in use by other hosts on the network link.{{Ref RFC|3927}}

This function can be dangerous from a cybersecurity viewpoint since an attacker can obtain information about the other hosts of its subnet to save in their ARP cache ([[ARP spoofing]]) an entry where the attacker MAC is associated, for instance, to the IP of the [[default gateway]], thus allowing them to [[Man-in-the-middle attack|intercept]] all the traffic to external networks.