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.

Format of the multicast address

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 ...

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