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ARP Address Specification and General Operation
(Page 1 of 2)
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.
ARP Message Types and Address Designations
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
Its a bit confusing, but youll
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
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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