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1.7 High-Level Protocols and Ethernet Addresses

Operation of the ARP Protocol

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The operation of ARP is straightforward. Let's say an IP-based station (station "A") with IP address 192.0.2.1 wishes to send data over the Ethernet channel to another IP-based station (station "B") with IP address 192.0.2.2. Station "A" sends a packet to the broadcast address containing an ARP request. The ARP request basically says "Will the station on this Ethernet channel that has the IP address of 192.0.2.2 please tell me what the address of its Ethernet interface is?"

Since the ARP request is sent in a broadcast frame, every Ethernet interface on the network reads it in and hands the ARP request to the networking software running on the station. Only station "B" with IP address 192.0.2.2 will respond, by sending a packet containing the Ethernet address of station "B" back to the requesting station. Now station "A" has an Ethernet address to which it can send data destined for station "B," and the high-level protocol communication can proceed.

A given Ethernet system can carry several different kinds of high-level protocol data. For example, a single Ethernet can carry data between computers in the form of TCP/IP protocols as well as Novell or AppleTalk protocols. The Ethernet is simply a trucking system that carries packages of data between computers; it doesn't care what is inside the packages.

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Quick Reference Guide to the Ethernet System - 04 SEP 95

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