Multicast Addresses

IPv4 Multicast Addresses

IPv4 addresses are classified as Class A, B, C, D, or E. Class D addresses are IPv4 multicast addresses and are carried in packets' destination address fields to identify multicast groups.

A multicast packet's source address field is a Class A, B, or C unicast address. A Class D address cannot be a source IP address in a multicast packet. Class E addresses are reserved for future use.

All receivers of a multicast group are identified by the same IPv4 multicast group address on the network layer. Once a user joins the group, the user can receive all IP packets sent to the group.

Class D addresses are in the 224.0.0.0 to 239.255.255.255 range. For details, see Table 1.

• Permanent multicast group addresses, also known as reserved multicast group addresses, are reserved by the Internet Assigned Number Authority (IANA) for routing protocols and remain unchanged. Each permanent multicast group address identifies all devices in a multicast group that may contain any number (including 0) of members. For details, see Table 2.

• A temporary multicast group address, also known as a common group address, is an IPv4 address that is assigned to a multicast group temporarily. If there is no user in this group, this address is reclaimed.

IPv6 Multicast Addresses

Figure 1 IPv6 multicast address format

Figure 1 shows the format of an IPv6 multicast address.

• An IPv6 multicast address starts with FF.

• The flags field includes four bits (0, R, P, and T).

  • 0: indicates the most significant bit, which is reserved and has a fixed value of 0.
  • R: indicates whether the multicast address is embedded with an RP address. If the value is 1, the multicast address is embedded with an RP address.
  • P: indicates whether the address is a unicast prefix-based multicast address. If the value is 1, the address is a unicast prefix-based multicast address.
  • T: indicates whether a multicast address is a permanent multicast group address. If the value is 0, the address is a multicast address is a permanent multicast group address or a well-known multicast address defined by the IANA.
• The scope field (4 bits) indicates whether a multicast group contains any node in the global address space or only the nodes of the same local network, the same site, or the same organization. Values in this field are defined as follows:

  • 0: reserved for other multicast protocol usage

  • 1: node/interface-local scope

  • 2: link-local scope

  • 3: reserved for other multicast protocol usage

  • 4: admin-local scope

  • 5: site-local scope

  • 8: organization-local scope

  • E: global scope

  • F: reserved for other multicast protocol usage

  • Any other value: unassigned and can be used as a common address

Table 3 shows the scopes and meanings of fixed IPv6 multicast addresses.

Multicast MAC Addresses

Multicast MAC addresses identify receivers of the same multicast group at the link layer.

Ethernet interface boards can identify multicast MAC addresses. After a multicast MAC address of a multicast group is configured on a device's driver, the device can then receive and forward data of the multicast group on the Ethernet. The mapping between the multicast IPv4 address and multicast IPv4 MAC address is as follows:

As defined by the IANA, the 24 most significant bits of a MAC address are 0x01005e, the 25th bit is 0, and the 23 least significant bits are the same as those of a multicast IPv4 address. Figure 2 shows the mapping between multicast IPv4 addresses and multicast MAC addresses.

Figure 2 Mapping between multicast IPv4 addresses and multicast MAC addresses

The first four bits of an IPv4 multicast address, 1110, are mapped to the 25 most significant bits of a multicast MAC address. In the last 28 bits, only 23 bits are mapped to a MAC address, resulting in the loss of 5 bits. Therefore, 32 IPv4 multicast addresses are mapped to the same MAC address.

As defined by the IANA, the higher-order 16 bits of an IPv6 MAC address are 0x3333, and the low-order 32 bits of an IPv6 MAC address are the same as those of a multicast IPv6 address. Figure 3 shows the mapping between multicast IPv6 addresses and multicast IPv6 MAC addresses.

Figure 3 Mapping between multicast IPv6 addresses and multicast MAC addresses

This document focuses on IP multicast technology and device operation. Multicast in the document refers to IP multicast, unless otherwise specified.