Octet

Say this first

Multicast addresses start with ff. The next 4 bits are flags and the 4 after that are the scope, so ff02:: is link scope and ff05:: is site scope. On a LAN you see ff02::1 all nodes, ff02::2 all routers, ff02::5 and ff02::6 for OSPFv3, ff02::1:2 for DHCPv6 and the solicited-node groups ff02::1:ffXX:XXXX. On Ethernet each group maps to MAC 33:33 plus the low 32 bits of the address.

Go deeper

Format

Output

| 8 bits | 4 bits | 4 bits |       112 bits      |
|  ff    | flags  | scope  |      group ID       |

Flags: T=0 means a permanent, well-known group; T=1 a transient one. The P and R flags mark prefix-based and embedded-RP addresses used in multicast routing.

Scope value	Scope
1	interface-local
2	link-local
4	admin-local
5	site-local
8	organization-local
e	global

Groups worth knowing by heart

Address	Group
`ff02::1`	all nodes
`ff02::2`	all routers
`ff02::5`	OSPFv3 all SPF routers
`ff02::6`	OSPFv3 DR and BDR
`ff02::9`	RIPng
`ff02::a`	EIGRP
`ff02::d`	PIM
`ff02::12`	VRRP
`ff02::16`	MLDv2 reports
`ff02::1:2`	all DHCPv6 relay agents and servers
`ff05::1:3`	all DHCPv6 servers (site scope)
`ff02::1:ff00:0/104`	solicited-node groups

MAC mapping

33:33 followed by the last 32 bits of the group. ff02::1 becomes 33:33:00:00:00:01, and ff02::1:ff3c:4d5e becomes 33:33:ff:3c:4d:5e. Only 32 of the 112 group bits make it into the MAC, so several groups can share one MAC. The NIC lets them all in and the IP stack does the final check.

They will ask next

Can a packet to ff02::2 cross a router?
No. Scope 2 is link-local. Routers never forward it, whatever the hop limit says.
What joins a router to ff02::2?
Enabling IPv6 routing. On Cisco that is ipv6 unicast-routing. Without it the box behaves as a host, does not join ff02::2 and does not send RAs.

Traps

Saying ff02::2 reaches all routers in the network. Only the ones on the link.
Mixing up the IPv4 multicast MAC prefix 01:00:5e with the IPv6 one 33:33.

How did you do?Book 4.3 Multicast Book 4.2 IP Version 6