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Address Resolution Protocol (ARP)

Recall that IP datagrams are forwarded over networks from host to router, router to router and router to host, and that every host and router has (at least one) unique IP address. From today's lecture, we also see that they also have a separate, unique MAC address -- used to address frames in the network.
 
The Address Resolution Protocol provides a mapping between IP addresses and MAC addresses. For example, in the case of IP local delivery (see earlier) the router or host knows the IP address of the destination machine, and also knows (from examination of the network/subnet address) that it is connected to the same network. In order to deliver the datagram, it:
 
  1. First broadcasts an ARP-request. In effect, the broadcast asks (eg) "Which one of you guys is 149.144.21.60?".
     
  2. The host whose IP address is 149.144.21.60 replies with its MAC address.
     
  3. The datagram is then encapsulated into a frame with the correct destination MAC address and placed "on the wire". The destination system notes its own MAC address as the frame's destination and picks up the frame, delivering the datagram to the IP software.

 
Systems which use IP keep an ARP cache of recent IP-to-MAC mappings to avoid the need for repeated ARP-requests. This "ARP Table" can usually be examined by the system manager. ARP is considered to be the "last hop" routing protocol for IP packets.
 
Lecture 13: Multiaccess Networks Copyright © 2004 P.Scott, La Trobe University Bendigo.



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