The Address Resolution Protocol (ARP)

The Address Resolution Protocol (ARP)
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Introduction

Purpose:
- Get the link (or access) layer address (e.g., MAC address) for a given network layer address (e.g., IP address)
Why?
- Suppose an Ethernet host wants to send to another host on the LAN -- it can use DNS to get the IP address of the recipient but then what?
- Suppose an Ethernet host on LAN 1 wants to send to an Ethernet host on LAN 2 -- it can use DNS to get the IP address of the recipient but how does it get the MAC address of the router?
Details:
- RFC826

ARP Message Contents

The Process

A check's its ARP table for B's IP address
If B's IP address is not in the table, A broadcasts an ARP request (to ff:ff:ff:ff:ff:ff)
B (or the router, if the IP address is not on the LAN) sends an ARP response to A (and adds A's IP-MAC pair to its ARP table)
A adds the B's information to its ARP table

Gratuitous ARP Messages

What are they?
- A way for a machine to announce that its IP or MAC address has changed
Two Approaches:
- A request in which the target's protocol address is the same as the sender's protocol address and the target's hardware address is 0
- A response in which the target's protocol address is the same as the sender's protocol address and the target's hardware address is the same as the sender's hardware address

Reverse Address Resolution Protocol (RARP)

Purpose:
- Get the network layer address (e.g., IP address) for a given link (or access) layer address (e.g., MAC address)
Why?
- Mostly useful for diskless workstations that have no way to store their IP addresses
Details:
- RFC903