Internet Core Protocols: The Definitive Guide

The Address Resolution Protocol

Since two IP devices on the same physical medium communicate with each other using the low-level protocols specific to that physical medium, the two devices must locate each other’s hardware addresses before they can exchange any data. However, each networking topology has its own addressing mechanisms that are different from all of the others, and IP has to be able to locate hardware addresses for each of them.

Since there are so many different types of networking topologies, it is not possible for IP to be imbued with the specific knowledge of how to build the address mappings for each of them explicitly. Attempting to do so would be an extraordinarily inefficient use of the IP software’s basic functionality, preventing the rapid adoption of new networking topologies and introducing other fundamental problems into the network.

Instead, the Address Resolution Protocol (ARP) is used as a helper to IP, and is called upon to perform the specific task of building each address mapping whenever address conversion is required. ARP works by issuing a broadcast on the selected medium, requesting that the device using a specific IP address respond with its hardware address. Once the destination device has responded, the sending system can establish communication with the receiving system and start sending data to the discovered hardware address. This process is shown in Figure 1.9, with 192.168.10.10 issuing a lookup for 192.168.10.40, who responds with its ...