Address Resolution Protocol

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.

A device on an Ethernet LAN uses two important addresses:

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

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.

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.

ARP uses two basic messages:

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.

IPv6 does not use ARP. It uses Neighbor Discovery Protocol (NDP), which relies on ICMPv6 messages.

Neighbor Discovery supports:

• Address resolution
• Router discovery
• Neighbor reachability checks
• Duplicate address detection
• Redirect messages

For address resolution, IPv6 uses Neighbor Solicitation and Neighbor Advertisement messages instead of ARP requests and replies.