Inverse Address Resolution Protocol (InARP)

Inverse Address Resolution Protocol (Inverse ARP or InARP), is a protocol used for obtaining Network Layer addresses of other nodes from Data Link Layer addresses. For example, in Ethernet networks InARP would primarily be used to get IP addresses when MAC addresses are known.

It is primarily used in Frame Relay and ATM networks, in which Layer 2 addresses of virtual circuits are sometimes obtained from Layer 2 signaling, and the corresponding Layer 3 addresses must be available before these virtual circuits can be used. In Frame Relay networks InARP is used to get Data Link Connection Identifier (DLCI) mappings to Virtual Circuits and is enabled automatically by default.

InARP operates essentially the same as ARP with the exception that InARP does not broadcast requests but sends them by unicast directly to the destination.  This is because the hardware address of the destination host is already known. The source host that sends the request simply formats it by inserting its source hardware and protocol addresses and the known destination hardware address. It then zero fills the target protocol address field.  Finally, it will encapsulate the packet for the specific network and send it directly to the target station. The diagram below gives us an illustration of how Inverse ARP works

1. Host A wants to send a packet destined to a host with MAC address CC-CC-CC-CC-CC-CC but does not know the IP address of the device
2. Host A then sends a unicast ARP request to the specified destination host
3. Switch A looks at the destination MAC address in the packet and forwards the packet to Host C
4. Host C updates its ARP table and sends a reply back to Host A
5. Host A receives the reply and updates its ARP table and then sends the packet unicast to Host C with its destination IP address

Upon receiving an InARP request, a station may put the source protocol address/hardware address mapping into its ARP cache as it would any ARP request.  Unlike other ARP requests, however, the destination host may assume that any InARP request it receives is destined for it. For every InARP request, the destination host may format a proper reply using the source addresses from the request as the target addresses of the reply.  If the host is unable or unwilling to reply, it ignores the request.

When the source station receives the InARP reply, it may complete the ARP table entry and use the provided address information. The information learned via InARP or ARP may be aged or invalidated under certain circumstances.

InARP (Layer 2 known, Layer 3 unknown) is the inverse of ARP (Layer 3 known, Layer 2 unknown) as they both do just the opposite. In addition, InARP is actually implemented as a protocol extension to ARP. InARP uses the same packet format as ARP; but has different operation codes.