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The TCP/IP Guide

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Table Of Contents  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)

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ARP Overview, Standards and History
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ARP Message Format
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ARP Address Specification and General Operation
(Page 2 of 2)

ARP General Operation

With that background in place, let's look at the steps followed in an ARP transaction (which are also shown graphically in the illustration in Figure 48):

Figure 48: Address Resolution Protocol (ARP) Transaction Process

This diagram shows the sequence of steps followed in a typical ARP transaction, as well as the message exchanges between a source and destination device, and cache checking and update functions. (Those little columns are supposed to be hard disks, not cans of soup! J)


  1. Source Device Checks Cache: The source device will first check its cache to determine if it already has a resolution of the destination device. If so, it can skip to the last step of this process, step #9.

  2. Source Device Generates ARP Request Message: The source device generates an ARP Request message. It puts its own data link layer address as the Sender Hardware Address and its own IP address as the Sender Protocol Address. It fills in the IP address of the destination as the Target Protocol Address. (It must leave the Target Hardware Address blank, since that it is what it is trying to determine!)

  3. Source Device Broadcasts ARP Request Message: The source broadcasts the ARP Request message on the local network.

  4. Local Devices Process ARP Request Message: The message is received by each device on the local network. It is processed, with each device looking for a match on the Target Protocol Address. Those that do not match will drop the message and take no further action.

  5. Destination Device Generates ARP Reply Message: The one device whose IP address matches the contents of the Target Protocol Address of the message will generate an ARP Reply message. It takes the Sender Hardware Address and Sender Protocol Address fields from the ARP Request message and uses these as the values for the Target Hardware Address and Target Protocol Address of the reply. It then fills in its own layer two address as the Sender Hardware Address and its IP address as the Sender Protocol Address. Other fields are filled in as explained in the topic describing the ARP message format.

  6. Destination Device Updates ARP Cache: If the source needs to send an IP datagram to the destination now, it makes sense that the destination will probably need to send a response to the source at some point soon. (After all, most communication on a network is bidirectional.) As an optimization, then, the destination device will add an entry to its own ARP cache containing the hardware and IP addresses of the source that sent the ARP Request. This saves the destination from needing to do an unnecessary resolution cycle later on.

  7. Destination Device Sends ARP Reply Message: The destination device sends the ARP reply message. This reply is, however, sent unicast to the source device, as there is no need to broadcast it.

  8. Source Device Processes ARP Reply Message: The source device processes the reply from the destination. It stores the Sender Hardware Address as the layer two address of the destination, to use for sending its IP datagram.

  9. Source Device Updates ARP Cache: The source device uses the Sender Protocol Address and Sender Hardware Address to update its ARP cache for use in the future when transmitting to this device.

Key Concept: ARP is a relatively simple request/reply protocol. The source device broadcasts an ARP Request looking for a particular device based on its IP address. That device responds with its hardware address in an ARP Reply message.

Note that this description goes a bit beyond the basic steps in address resolution, because two enhancements are mentioned. One is caching, which is described in its own topic but had to be mentioned here because it is the first step in the process, for obvious reasons. The other is cross-resolution (described in the overview of caching issues in dynamic resolution), which is step #6 of the process. This is why the source device includes its IP address in the request. It isn't really needed for any other reason, so you can see that this feature was built into ARP from the start.

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ARP Overview, Standards and History
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ARP Message Format
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