18.1.5 Multicast address

A multicast address identifies a group of nodes. The upper 8 bits of the address format prefix are ff. A node can belong to multiple multicast groups. Multicast addresses cannot be used as the source addresses of packets. In a multicast address, the address format prefix is followed by the flag field (4 bits), scope field (4 bits), and group ID field (112 bits). The following figure shows the format of an IPv6 multicast address.

Figure 18-18: IPv6 Multicast address

Each of the four bits in the flag field is a flag. The fourth bit is defined as the transient (T) flag bit, and its values have the following meaning:

If the T flag bit is set to 0: The address is a known multicast address permanently assigned by IANA.
If the T flag bit is set to 1: The address is a temporarily used (non-permanent) multicast address

The scope field is a 4-bit flag and used to limit the scope of a multicast group. The following table describes the scope field values for multicast addresses.

Table 18-2: Scope field values for multicast addresses
Code	Scope
0	Reserved
1	Node-local
2	Link-local
3	Unassigned
4	Admin local scope
5	Site-local
6	Unassigned
7	Unassigned
8	Organization-local
9	Unassigned
A	Unassigned
B	Unassigned
C	Unassigned
D	Unassigned
E	Global
F	Reserved

<Structure of this section>

(1) Reserved multicast address

The following multicast addresses are reserved and cannot be assigned to any multicast group.

ff00:0:0:0:0:0:0:0
ff01:0:0:0:0:0:0:0
ff02:0:0:0:0:0:0:0
ff03:0:0:0:0:0:0:0
ff04:0:0:0:0:0:0:0
ff05:0:0:0:0:0:0:0
ff06:0:0:0:0:0:0:0
ff07:0:0:0:0:0:0:0
ff08:0:0:0:0:0:0:0
ff09:0:0:0:0:0:0:0
ff0a:0:0:0:0:0:0:0
ff0b:0:0:0:0:0:0:0
ff0c:0:0:0:0:0:0:0
ff0d:0:0:0:0:0:0:0
ff0e:0:0:0:0:0:0:0
ff0f:0:0:0:0:0:0:0

(2) All node addresses

An all-nodes address identifies a group of IPv6 nodes in the specified scope. When the destination address of a packet is an all-nodes address, the packet is received by all the nodes in the specified scope. The following all-nodes addresses are available:

ff01:0:0:0:0:0:0:1 Node-local all-nodes address
ff02:0:0:0:0:0:0:1 Link-local all-nodes address

(3) All router addresses

An all-routers address identifies a group of IPv6 routers in the specified scope. When the destination address of a packet is an all-routers address, the packet is received by all the routers in the specified scope. The following all-routers addresses are available:

ff01:0:0:0:0:0:0:2 Node-local all-routers address
ff02:0:0:0:0:0:0:2 Link-local all-routers address
ff05:0:0:0:0:0:0:2 Site-local all-routers address

(4) Solicited node address

A solicited-node address is created by using the unicast address or anycast address of a node. The lower 24 bits in the unicast or anycast address of the solicited node are added to the 104-bit prefix, ff02:0:0:0:0:1:ff00::/104. The range of solicited-node addresses is as follows:

ff02:0:0:0:0:1:ff00:0000 ~ ff02:0:0:0:0:1:ffff:ffff

If solicited-node addresses need to be created from IPv6 addresses that have a few different upper bits and those bits are different only because each provider of aggregation identifiers uses different bits in the format prefix, all such solicited-node addresses are the same. This decreases the number of multicast groups that nodes need to participate in.