RFARP and RARP Protocol Basics
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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 Position | Description | ARP Request (Received) - Hex | ARP Response (Transmitted) - Hex |
|---|---|---|---|
| 1-6 | Broadcast address (ARP REQ) / Ethernet destination address (ARP RES) | FF FF FF FF FF FF | 00 A0 C9 22 89 56 |
| 7-12 | Source Ethernet address | 00 A0 C9 22 89 56 | 00 A0 C9 E7 31 45 |
| 13-14 | Type field | 08 06 | 08 06 |
| 15-16 | Ethernet | 00 01 | 00 01 |
| 17-18 | Protocol type | 08 00 | 08 00 |
| 19 | HA length | 06 | 06 |
| 20 | PA length | 04 | 04 |
| 21-22 | Operation (1: ARP REQ, 2: ARP RES) | 00 01 | 00 02 |
| 23-28 | Sender Ethernet address | 00 A0 C9 22 89 56 | 00 A0 C9 E7 31 45 |
| 29-32 | Sender protocol address | AC 12 02 47 | AC 12 02 3F |
| 33-38 | Target Ethernet address | 00 00 00 00 00 00 | 00 A0 C9 22 89 56 |
| 39-42 | Target Protocol address | AC 12 02 3F | AC 12 02 47 |
| 43-60 | PAD | 7E (all bytes) | 7E (all bytes) |
