How does ICMPv6 Neighbor Solicitation/Advertisement work for address resolution?

To find a neighbor's MAC, a device sends an NS (carrying the target IPv6 address) to the solicited-node multicast address; the owner of that address replies with an NA containing its MAC. This replaces IPv4 ARP.

ICMPv6 Address Resolution (NS/NA):

ICMPv6 uses Neighbor Solicitation and Neighbor Advertisement messages to resolve IPv6 addresses to MAC addresses (replacing ARP from IPv4).

* The requester multicasts an NS for the target IPv6; only the owner answers with an NA giving its MAC — more targeted than IPv4 ARP broadcasts. *

Process:

Router R1                         Host PC1
(needs MAC for PC1)              (2001:db8:acad:1::10)
     │                               │
     │─── NS: "Who has ::10?" ──────→│
     │    (to solicited-node multicast)
     │                               │
     │←── NA: "I do! MAC=00:aa:bb..."│
     │                               │

Steps:

1. R1 needs to send a frame to PC1's IPv6 address
2. R1 sends NS to the solicited-node multicast address
3. NS includes the target IPv6 address
4. PC1 recognizes its address and responds with NA
5. NA contains PC1's MAC address
6. R1 caches the mapping and sends the frame

Key difference from ARP:

• ARP uses broadcast (all hosts process)
• ICMPv6 NS uses solicited-node multicast (only target processes)
• More efficient than broadcast

Go deeper:

• RFC 4861 — Neighbor Discovery for IP version 6 (IPv6) — defines NS/NA address resolution and the solicited-node multicast mechanism.
• Neighbor Discovery Protocol — Wikipedia — how NS/NA replaces ARP for IPv6 link-layer resolution.