(InARP) Additions to ARP typically used for Frame Relay. [Any other examples of its use?]
Frame Relay stations route frames of a higher level protocol between LANs, across a Permanent Virtual Circuit. These stations are identified by their Data Link Control Identifier (DLCI), equivalent to an Ethernet address in a LAN itself.
InARP allows a station to determine a protocol address (e.g. IP address) from a DLCI. This is useful if a new virtual circuit becomes available. Signalling messages announce its DLCI, but without the corresponding protocol address it is unusable: no frames can be routed to it.
Reverse ARP (RARP) performs a similar task on an Ethernet LAN, however RARP answers the question "What is my IP Address?" whereas InARP answers the question "What is your protocol address?".
See RFC 2390.