Address Resolution Protocol (ARP) performs mapping of an IP address to a physical machine address (MAC address for Ethernet) that is recognized in the local network. For example, in IP Version 4, an address is 32 bits long. In an Ethernet local area network, however, addresses for attached devices are 48 bits long. A table, usually called the ARP cache, is used to maintain a correlation between each MAC address and its corresponding IP address. ARP provides the rules for making this correlation and providing address conversion in both directions.

Since protocol details differ for each type of local area network, there are separate ARP specifications for Ethernet , Frame Relay , ATM , Fiber Distributed-Data Interface, HIPPI, and other protocols.

Inverse Address Resolution Protocol (InARP) is an addition to ARP to address ARP in Frame Relay environment.

There is a Reverse ARP (RARP ) for host machines that don't know their IP address. RARP enables them to request their IP address from the gateway's ARP cache.Details of RARP is presented in a separate document.
Protocol Structure - ARP and InARP
ARP and InARP has the same structure:

16		32 bit
Hardware Type		Protocol Type
HLen	Plen	Operation
Sender Hardware Address
Sender Protocol Address
Target Hardware Address
Target Protocol Address

• Hardware type - Specifies a hardware interface type for which the sender requires a response.
• Protocol type - Specifies the type of high-level protocol address the sender has supplied.
• Hlen - Hardware address length.
• Plen - Protocol address length.
• Operation - The values are as follows:
1. ARP request.
2. ARP response.
3. RARP request.
4. RARP response.
5. Dynamic RARP request.
6. Dynamic RARP reply.
7. Dynamic RARP error.
8. InARP request.
9. InARP reply.
• Sender hardware address -HLen bytes in length.
• Sender protocol address - PLen bytes in length.
• Target hardware address - HLen bytes in length.
• Target protocol address - PLen bytes in length