IPv6 Essentials

Multicast Address

A multicast address is an identifier for a group of nodes, identified by the high-order byte FF, or 1111 1111 in binary notation (refer to Table 3-2). A node can belong to more than one multicast group. Multicast exists in IPv4, but it has been redefined and improved for IPv6. The multicast address format is shown in Figure 3-9.

Figure 3-9. Format of the multicast address

The first byte identifies the address as a multicast address. The next 4 bits are used for Flags, defined as follows: The first 3 bits of the Flag field must be zero; they are reserved for future use. The last bit of the Flag field indicates whether this address is permanently assigned—i.e., one of the well-known multicast addresses assigned by the IANA—or a temporary multicast address. A value of zero for the last bit defines a well-known address; a value of one indicates a temporary address. The Scope field is used to limit the scope of a multicast address. The possible values are shown in Table 3-5.

Table 3-5. Values for the Scope field

Value	Description
0	Reserved
1	Node-local scope (name changed to interface-local in new draft)
2	Link-local scope
3, 4	Unassigned
5	Site-local scope
6, 7	Unassigned
8	Organization-local scope
9, A, B, C, D	Unassigned
E	Global scope
F	Reserved

Well-Known Multicast Addresses

The last 112 bits of the address carry the multicast group ...