ARP - Address Resolution Protocol

Overview

The Address Resolution Protocol (ARP) allows for conversion from a network layer address to a hardware layer address (i.e. from the IP address to the MAC address).

Function

Prerequisites

  • Two systems that each know their own IP address and MAC address
  • A usable network layer path between the two systems
  • The sending system must know the IP address of the destination system

The sending system wants to communicate with the destination system, but only knows the IP address of the target. Communications at the hardware layer require a MAC address (or similar). To find the hardware layer address the sending system uses ARP.

The sending system transmits an ARP request that basically says "I'm MY.NET.ADD.RSS and I'm looking for the hardware address of SOM.NET.ADD.RSS. What is your hardware address?" Each system that receives the ARP packet then looks and sees if they match the requested address. If they do, they respond with their hardware address.

Full Breakout

IPv4

Bit Range Bit Use
0 - 15 Hardware Type
16 - 31 Protocol Type
32 - 39 Hardware Address Length
40 - 47 Protocol Address Length
48 - 63 Operation
64 - 111 Sender Hardware Address
112 - 143 Sender Protocol Address
144 - 191 Target Hardware Address
192 - 223 Target Protocol Address

Operation transmits a value of 1 for a request, and 2 for a reply.

RFCs

See Also

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