What is the IPv6 multicast prefix, and what are the two well-known multicast groups every link uses?
Every multicast address starts with the ff00::/8 prefix; the two well-known link-local groups are ff02::1 (all-nodes — every IPv6 device) and ff02::2 (all-routers — every IPv6 router).
A multicast address delivers one packet to a group of interested interfaces at once, and you spot one instantly because the first byte is ff. Of the many assigned groups, two link-local ones (ff02:: scope) carry almost all the day-to-day work and are worth memorising. ff02::1 is the all-nodes group — it stands in for what IPv4 called broadcast, since a packet sent there is processed by every IPv6 interface on the link, and every interface joins it automatically. ff02::2 is the all-routers group; only routers join it, and a Cisco device becomes a member exactly when you enable ipv6 unicast-routing. That is why a host's Router Solicitation is sent to ff02::2 (reach the routers) while a router's Router Advertisement goes to ff02::1 (reach all hosts).
| Address | Name | Members |
|---|---|---|
| ff02::1 | All-nodes | Every IPv6-enabled device on the link |
| ff02::2 | All-routers | Every IPv6 router on the link |
Gotcha: a multicast address can only ever be a packet's destination, never its source — a host always sources traffic from one of its own unicast addresses.
Go deeper:
Multicast address — IPv6 (Wikipedia) — the ff00::/8 prefix, scope field, and the well-known groups including ff02::1 / ff02::2.- RFC 4291 §2.7 — Multicast Addresses — multicast format, scopes and the predefined all-nodes / all-routers / solicited-node groups.