IPv4 Multicast Addresses

The Internet Assigned Numbers Authority (IANA) allocates Class D addresses for IPv4 multicast. An IPv4 address is 32 bits long, and the first four bits of a Class D IP address are 1110. Therefore, multicast IP addresses range from 224.0.0.0 to 239.255.255.255. Table 2-2 describes IPv4 multicast addresses.

IPv6 Multicast Addresses

An IPv6 address is 128 bits long. The IPv6 multicast address format is shown in Figure 2-4.

Figure 2-4  IPv6 multicast address format

An IPv6 multicast address has a Group ID field to identify a multicast group.

• FF: The leftmost eight bits are 11111111, indicating a multicast address. All IPv6 multicast addresses start with FF.

• Flags: This field is 4 bits long and identifies the state of a multicast address.

  Table 2-4  Description of flag values

  Value	Description
  0	Permanent multicast group addresses that are reserved by the IANA
  1	ASM group addresses
  2	ASM group addresses
  3	SSM group addresses
  Others	Unassigned

• Scope: This field is 4 bits long and identifies the scope of a multicast group, for example, whether a multicast group covers nodes in the same network, same site, same organization or any node in the global address space.

  Table 2-5  Description of Scope field values

  Value	Description
  0, 3, F	Reserved
  1	Node/Interface-local scope
  2	Link-local scope
  4	Admin-Local scope
  5	Site-local scope
  8	Organization-local scope
  E	Global scope
  Others	Unassigned

• Group ID: This field is 112 bits long and identifies a unique multicast group in the range specified by the Scope field. The Group ID can be permanently or temporarily assigned, depending on the value of the T flag in the Flags field.

Table 2-6 describes the IPv6 multicast address ranges.

IPv4 Multicast MAC Addresses

When unicast IPv4 packets are transmitted on an Ethernet network, the packets use MAC addresses of receivers as destination MAC addresses. However, the destination of a multicast data packet is a group with changeable members but not a specific receiver. Therefore, multicast data packets must use IPv4 multicast MAC addresses on an Ethernet network. IPv4 multicast MAC addresses are link-layer addresses mapped from IPv4 multicast addresses.

As defined by the IANA, the leftmost 24 bits of an IPv4 multicast MAC address are 0x01005E, the 25th bit is 0, and the rightmost 23 bits are mapped to the rightmost 23 bits of a multicast IPv4 address, as shown in Figure 2-5. Multicast MAC address 01-00-5e-00-01-01 is mapped to multicast IP address 224.0.1.1.

Figure 2-5  Mapping between an IPv4 multicast address and an IPv4 multicast MAC address

The first four bits of an IPv4 multicast address are fixed as 1110, mapping the leftmost 25 bits of a multicast MAC address. Among the last 28 bits, only 23 bits are mapped to a MAC address, and 5 bits are lost. As a result, 32 multicast IP addresses are mapped to one MAC address. For example, multicast IP addresses 224.0.1.1, 224.128.1.1, 225.0.1.1, and 239.128.1.1 are all mapped to multicast MAC address 01-00-5e-00-01-01. Address conflicts must be considered in address assignment.

IPv6 Multicast MAC Addresses

In an IPv6 multicast MAC address, the leftmost 16 bits are 0x3333, and the rightmost 32 bits are mapped to the rightmost 32 bits of an IPv6 multicast address. Figure 2-6 shows the mapping between IPv6 multicast address FF01::1111:1 and an IPv6 multicast MAC address.

Figure 2-6  Mapping between an IPv6 multicast address and an IPv6 multicast MAC address