GratuitousRequestVsAnnouncement

The difference between ARPAnnouncement and a GratuitousARPRequest is one of context (the packets are identical), with ARPAnnouncement being a specific use case of a GratuitousARPRequest that signals ARPDuplicateAddressDetection. @ref

In a reply to a comment, the author widens that: “Gratuitous ARP can be Opcode 1 or 2. ARP Announcements must be opcode 1.” @ref

RFC5227 introduced the new name partly because gratuitous ARP had become ambiguous, since people used it for both requests and replies. @ref

These notes treat ARPAnnouncement as a purpose and GratuitousARPRequest as the packets.

Wireshark’s own announcement flag is narrower still: a gratuitous request with Target MAC 00:00:00:00:00:00 @ref

Gratuitous ARP request is the older, more generic term. It describes the packet itself, sent for any reason at any time, such as a failover, a MAC change, or a periodic refresh. It doesn’t imply that any probing happened first. @ref