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 1–7 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. These flags must be initialized to zero.
T=0 – Indicates a permanently assigned, well-known, multicast address. This multicast address is assigned by the global Internet authority that assigns numbers.
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 1–8 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.