The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Network Interface / Internet "Layer Connection" Protocols            9  Address Resolution and the TCP/IP Address Resolution Protocol (ARP)                 9  TCP/IP Address Resolution Protocol (ARP)

ARP Overview, Standards and History 1 2 ARP Message Format

An Address Resolution Protocol transaction begins when a source device on an IP network has an IP datagram to send. It must first decide whether the destination device is on the local network or a distant network. If the former, it will send directly to the destination; if the latter, it will send the datagram to one of the routers on the physical network for forwarding. Either way, it will determine the IP address of the device that needs to be the immediate destination of its IP datagram on the local network. After packaging the datagram it will pass it to its ARP software for address resolution.

Basic operation of ARP is a request/response pair of transmissions on the local network. The source (the one that needs to send the IP datagram) transmits a broadcast containing information about the destination (the intended recipient of the datagram). The destination then responds unicast back to the source, telling the source the hardware address of the destination.

The terms source and destination apply to the same devices throughout the transaction. However, there are two different messages sent in ARP, one from the source to the destination and one from the destination to the source. For each ARP message, the sender is the one that is transmitting the message and the target is the one receiving it. Thus, the identity of the sender and target change for each message:

• Request: For the initial request, the sender is the source, the device with the IP datagram to send, and the target is the destination.
  
  
• Reply: For the reply to the ARP request, the sender is the destination; it replies to the source, which becomes the target.

It’s a bit confusing, but you’ll get used to it. J Each of the two parties in any message has two addresses (layer two and layer three) to be concerned with, so four different addresses are involved in each message:

• Sender Hardware Address: The layer two address of the sender of the ARP message.
  
  
• Sender Protocol Address: The layer three (IP) address of the sender of the ARP message.
  
  
• Target Hardware Address: The layer two address of the target of the ARP message.
  
  
• Target Protocol Address: The layer three (IP) address of the target.

These addresses each have a position in the ARP message format.

ARP Overview, Standards and History 1 2 ARP Message Format