BSIT Survival Kit

Introduction

Address resolution allows a device to match a logical Layer 3 address with the Layer 2 address needed for local delivery. In IPv4, this function is handled by Address Resolution Protocol (ARP). In IPv6, similar functions are handled by Neighbor Discovery.

MAC and IP Addresses

A device on an Ethernet LAN uses two important addresses:

Address	Layer	Purpose
MAC address	Layer 2	Local delivery from one network interface to another on the same link.
IP address	Layer 3	End-to-end logical addressing across networks.

When a device sends a packet, it needs an IP address for the final destination and a MAC address for the next local hop.

Destination on the Same Network

If the destination IP address is on the same local network, the sender needs the destination host's MAC address. If it does not already know the MAC address, it uses ARP to discover it.

Destination on a Remote Network

If the destination IP address is on another network, the sender does not send the frame directly to the remote host's MAC address. Instead, it sends the frame to the default gateway's MAC address. The router then forwards the packet toward the remote network.

How ARP Works

ARP uses two basic messages:

ARP Message	Purpose
ARP Request	Broadcast asking, “Who has this IPv4 address?”
ARP Reply	Unicast response containing the matching MAC address.

Devices store resolved addresses in an ARP table or ARP cache for a limited time. This reduces the need to send repeated ARP requests.

Diagram description: An ARP diagram often shows one host broadcasting a request to all local devices. Only the device with the matching IPv4 address replies with its MAC address.