Terminology » Networking Basics » ARP and RARP Protocol Basics

ARP and RARP Protocol Basics

arp rarp protocol networking mac address

This page covers the fundamentals of the ARP (Address Resolution Protocol) and RARP (Reverse Address Resolution Protocol) protocols. It provides an overview of their functionalities, along with examples illustrating the fields contained within ARP Request and ARP Response messages.

This tutorial provides a comprehensive guide to networking essentials, including:

  • Circuit Switching vs. Packet Switching
  • TCP/IP protocol fields
  • ARP/RARP protocol fields
  • What is an IP address?
  • What is a MAC address?
  • Networking devices: Hub, Switch, Bridge, Router, Gateway, and Firewall

ARP and RARP Overview

ARP (Address Resolution Protocol) is primarily used to determine the Ethernet (MAC) address of a device when its IP address is known. Imagine you need to send a letter to someone (IP address), but you only know their name (MAC address) – ARP helps you find their exact street address (MAC address).

RARP (Reverse Address Resolution Protocol) performs the opposite function of ARP. It helps a device discover its IP address when only its Ethernet address is known. This is useful for diskless workstations that need to obtain an IP address during startup.

ARP Request and Response Fields

The following table details the fields present in ARP Request and ARP Response messages:

ARP requests are broadcast packets sent to all devices on the network. The device whose IP address matches the target IP address in the broadcast packet will respond directly to the source device with its Ethernet address.

Byte PositionDescriptionARP Request (Received) - HexARP Response (Transmitted) - Hex
1-6Broadcast address (ARP REQ) / Ethernet destination address (ARP RES)FF FF FF FF FF FF00 A0 C9 22 89 56
7-12Source Ethernet address00 A0 C9 22 89 5600 A0 C9 E7 31 45
13-14Type field08 0608 06
15-16Ethernet00 0100 01
17-18Protocol type08 0008 00
19HA length0606
20PA length0404
21-22Operation (1: ARP REQ, 2: ARP RES)00 0100 02
23-28Sender Ethernet address00 A0 C9 22 89 5600 A0 C9 E7 31 45
29-32Sender protocol addressAC 12 02 47AC 12 02 3F
33-38Target Ethernet address00 00 00 00 00 0000 A0 C9 22 89 56
39-42Target Protocol addressAC 12 02 3FAC 12 02 47
43-60PAD7E (all bytes)7E (all bytes)