A multicast address is a logical identifier for a group of hosts in a computer network that are available to process datagrams or frames intended to be multicast for a designated network service. Multicast addressing can be used in the link layer (layer 2 in the OSI model), such as Ethernet multicast, and at the internet layer (layer 3 for OSI) for Internet Protocol Version 4 (IPv4) or Version 6 (IPv6) multicast.
IPv4 multicast addresses are defined by the most-significant bit pattern of 1110. This originates from the classful network design of the early Internet when this group of addresses was designated as Class D. The CIDR notation for this group is 224.0.0.0/4. The group includes the addresses from 224.0.0.0 to 239.255.255.255. Address assignments from within this range are specified in RFC 5771, an Internet Engineering Task Force (IETF) Best Current Practice document (BCP 51).
The address range is divided into blocks each assigned a specific purpose or behavior.
Notable IPv4 multicast addresses[edit]
The following table is a list of notable well-known IPv4 addresses that are reserved for IP multicasting and that are registered with the Internet Assigned Numbers Authority (IANA).
Multicast addresses in IPv6 use the prefix ff00::/8. IPv6 multicast addresses can be structured using the old format (RFC 2373) or the new format (RFC 3306, updated by RFC 7371).
General multicast address format (old)Bits844112Fieldprefixflagsscopegroup IDGeneral multicast address format (new)Bits8444486432Fieldprefixff1scopeff2reservedplennetwork prefixgroup IDThe prefix holds the value ff for all multicast addresses.
Currently, 3 of the 4 flag bits in the flags field (ff1) are defined; the most-significant flag bit is reserved for future use. The other three flags are known as R, P and T. All 4 bits in the additional flags field (ff2) are reserved for future use and must hold the value 0.
Multicast address flagsBitFlag010 (MSB)Reserved(Reserved)(Reserved)1R (Rendezvous)Rendezvous point not embeddedRendezvous point embedded2P (Prefix)Without prefix informationAddress based on network prefix3 (LSB)T (Transient)Well-known multicast addressDynamically assigned multicast addressSimilar to a unicast address, the prefix of an IPv6 multicast address specifies its scope, however, the set of possible scopes for a multicast address is different. The 4-bit sc (or scope) field (bits 12 to 15) is used to indicate where the address is valid and unique.
Multicast address scopeIPv6 addressIPv4 equivalentScopePurposeffx0::/16, ffxf::/16Reservedffx1::/16Interface-localPackets with this destination address may not be sent over any network link, but must remain within the current node; this is the multicast equivalent of the unicast loopback address.ffx2::/16224.0.0.0/24Link-localPackets with this destination address may not be routed anywhere.ffx3::/16239.255.0.0/16Realm-Local scopeLocal multicast particular to a network technologyffx4::/16Admin-localThe smallest scope that must be administratively configured.ffx5::/16Site-localRestricted to the local physical network.ffx8::/16239.192.0.0/14Organization-localRestricted to networks used by the organization administering the local network. (For example, these addresses might be used over VPNs; when packets for this group are routed over the public internet (where these addresses are not valid), they would have to be encapsulated in some other protocol.)ffxe::/16224.0.1.0-238.255.255.255Global scopeEligible to be routed over the public internet.The service is identified in the group ID field. For example, if ff02::101 refers to all Network Time Protocol (NTP) servers on the local network segment, then ff08::101 refers to all NTP servers in an organization's networks. The group ID field may be for special multicast address types.
Notable IPv6 multicast addresses[edit]
The following table is a list notable IPv6 multicast addresses that are registered with IANA.
Ethernet[edit]
Ethernet frames with a value of 1 in the least-significant bit of the first octet of the destination MAC address are treated as multicast frames and are flooded to all points on the network. While frames with ones in all bits of the destination address (FF-FF-FF-FF-FF-FF) are sometimes referred to as broadcasts, Ethernet generally does not distinguish between multicast and broadcast frames. Modern Ethernet controllers filter received packets to reduce CPU load, by looking up the hash of a multicast destination address in a table, initialized by software, which controls whether a multicast packet is dropped or fully received.
The IEEE has allocated the address block 01-80-C2-00-00-00 to 01-80-C2-FF-FF-FF for group addresses for use by standard protocols. Of these, the MAC group addresses in the range of 01-80-C2-00-00-00 to 01-80-C2-00-00-0F are not forwarded by 802.1D-conformant MAC bridges.