Gratuitous ARP

Gratuitous ARP is a special ARP (Address Resolution Protocol) reply that is not a response to an ARP request. A Gratuitous ARP reply is a reply to without a ARP request. No reply is expected for a Gratuitous ARP . A Gratuitous ARP packet has the following characteristics.

• The source and destination IP Addresses are both set to the IP of the machine sending the Gratuitous ARP packet.
• Destination MAC address is the broadcast MAC address ff:ff:ff:ff:ff:ff.

Gratuitous ARP packets are generated by network devices for some of the reasons listed below.

• To detect duplicate IPv4 addresses. When a reply to a gratuitous ARP request is received, computers can detect IPv4 address conflict in the network.
• To update ARP tables after a IPv4 address or MAC address change

Following Wireshark capture screenshot shows a Gratuitous ARP packet.