RARP is the protocol that is used when a device boots up without an IP address and requests an IP address. RARP is typically not used in today's networks, and is replaced by DHCP.
Inverse Address Resolution Protocol (InARP) is an addition to ARP which addresses ARP in a Frame Relay environment. InARP discovers the remote end's data-link connection identifier (DLCI).
A gratuitous ARP is when the MAC address in a system is changed. That is, the MAC address for a given Hosts IP address mapping is changed for any valid reason, such as network card replacement or router failure. In this case, when the host or router is rebooted or replaced, the device sends a gratuitous ARP packet advising all hosts of the new MAC address. Because this is a broadcast packet, all the hosts in the network receive and process this packet. They update their old mapping in the ARP cache with this new mapping. This ensures that devices can communicate immediately.
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