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

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.^[1] 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.

Local subnetwork

Addresses in the range of 224.0.0.0 to 224.0.0.255 are individually assigned by IANA and designated for multicasting on the local subnetwork only. For example, the Routing Information Protocol (RIPv2) uses 224.0.0.9, Open Shortest Path First (OSPF) uses 224.0.0.5 and 224.0.0.6, and Multicast DNS uses 224.0.0.251. Routers must not forward these messages outside the subnet from which they originate.

Internetwork control block

Addresses in the range 224.0.1.0 to 224.0.1.255 are individually assigned by IANA and designated as the internetwork control block. This block of addresses is used for traffic that must be routed through the public Internet, such as for applications of the Network Time Protocol using 224.0.1.1.

AD-HOC block

Addresses in three separate blocks are not individually assigned by IANA. These addresses are globally routed and are used for applications that don't fit either of the previously described purposes.^[9]

Source-specific multicast

The 232.0.0.0/8 (IPv4) and ff3x::/32 (IPv6) blocks are reserved for use by source-specific multicast.

GLOP^[10]

The 233.0.0.0/8 range was originally assigned by RFC 2770 as an experimental, public statically-assigned multicast address space for publishers and Internet service providers that wished to source content on the Internet. The allocation method is termed GLOP addressing and provides implementers a block of 255 addresses that is determined by their 16-bit autonomous system number (ASN) allocation. In a nutshell, the middle two octets of this block are formed from assigned ASNs, giving any operator assigned an ASN 256 globally unique multicast group addresses.^[11] The method is not applicable to the newer 32-bit ASNs. RFC 3180, superseding RFC 2770, envisioned the use of the range for many-to-many multicast applications. Unfortunately, with only 256 multicast addresses available to each autonomous system, GLOP is not adequate for large-scale broadcasters.^{[citation needed]}

Unicast-prefix-based

The 234.0.0.0/8 range is assigned by RFC 6034 as a range of global IPv4 multicast address space provided to each organization that has /24 or larger globally routed unicast address space allocated; one multicast address is reserved per /24 of unicast space. A resulting advantage over GLOP is that the unicast-prefix mechanism resembles the unicast-prefix capabilities of IPv6 as defined in RFC 3306.

Administratively scoped

The 239.0.0.0/8 range is assigned by RFC 2365 for private use within an organization. Per the RFC, packets destined to administratively scoped IPv4 multicast addresses do not cross administratively defined organizational boundaries, and administratively scoped IPv4 multicast addresses are locally assigned and do not have to be globally unique. The RFC also discusses structuring the 239.0.0.0/8 range to be loosely similar to the scoped IPv6 multicast address range described in RFC 1884.

Ethernet-specific

In support of link-local multicasts which do not use IGMP, any IPv4 multicast address that falls within the *.0.0.0/24 and *.128.0.0/24 ranges will be broadcast to all ports on many Ethernet switches, even if IGMP snooping is enabled, so addresses within these ranges should be avoided on Ethernet networks where the functionality of IGMP snooping is desired.^{[12][dubious – discuss]}

Notable IPv4 multicast addresses

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).^[5]

IPv6

Multicast addresses in IPv6 use the prefix ff00::/8. The general format of the IPv6 multicast address is described by RFC 4291:

Based on the value of the flag bits, IPv6 multicast addresses can be Unicast-Prefix-based Multicast Addresses, Source-Specific Multicast Addresses (both types introduced by RFC 3306, updated by RFC 7371), or Embedded RP IPv6 Multicast Addresses (introduced by RFC 3956, updated by RFC 7371). Each of these types of multicast addresses have their own format and follow specific rules.

Similar 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 scope field (bits 12 to 15) is used to indicate where the address is valid and unique.

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 further divided for special multicast address types.

Notable IPv6 multicast addresses

The following table is a list notable IPv6 multicast addresses that are registered with IANA.^[21] To be included in some of the below multicast groups a client must send a Multicast Listener Discovery (MLD), a component of ICMPv6 suite, to join that group.^[22] For example, to listen to ff02::1:ff28:9c5a, a client must send a MLD report to the router, containing the multicast address, to indicate that it wants to listen to that group.^[23]

Ethernet

Ethernet frames with a value of 1 in the least-significant bit of the first octet^{[note 3]} 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.^[27]

802.11

802.11 wireless networks use the same MAC addresses for multicast as Ethernet.