Multicast Addresses

An IPv6 multicast address is an identifier for a group of interfaces. An interface can belong to any number of multicast groups. The following table shows the multicast address format.

Table 14–7 Multicast Address Format

8 bits	4 bits	4 bits	112 bits
11111111	FLGS	SCOP	Group ID

11111111 at the start of the address identifies the address as a multicast address. FLGS is a set of four flags: 0,0,0,T.

The high-order 3 flags are reserved and must be initialized to zero.

• T=0 – Indicates a permanently assigned (well-known) multicast address, assigned by the global Internet numbering authority.

• T=1 – Indicates a non-permanently assigned (transient) multicast address.

SCOP is a 4-bit multicast scope value used to limit the scope of the multicast group. The following table shows the SCOP values.

Table 14–8 SCOP Values

0	Reserved	8	Organization-local scope
1	Node-local scope	9	(unassigned)
2	Link-local scope	A	(unassigned)
3	(unassigned)	B	(unassigned)
4	(unassigned)	C	(unassigned)
5	Site-local scope	D	(unassigned)
6	(unassigned)	E	Global scope
7	(unassigned)	F	Reserved

Group ID identifies the multicast group, either permanent or transient, within the given scope.