Multicast address

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 leading address bits of 1110, originating from the classful network design of the early Internet when this group of addresses was designated as Class D. The Classless Inter-Domain Routing (CIDR) prefix of 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.

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 Zeroconf mDNS uses 224.0.0.251. Routers must not forward these messages outside the subnet in 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 (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.^[5]

Source-specific multicast

The 232.0.0.0/8 (IPv4) and FF3x::/32 (IPv6) block is reserved for use by source-specific multicast.

GLOP

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. The method is not applicable to the newer 32-bit extension AS numbers. RFC 3180, superseding RFC 2770, envisioned the use of the range for many-to-many multicast applications. This block has been one of the most successful multicast addressing schemes. Unfortunately, with only 256 multicast addresses available to each autonomous system, GLOP is not adequate for large-scale broadcasters.^[6]

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 mechanisms in IPv4 and IPv6 become more similar.

Administratively scoped

The 239.0.0.0/8 range is assigned by RFC 2365 for private use within an organization. From 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.

Notable 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).^[7]

IPv6

Multicast addresses in IPv6 use the prefix ff00::/8. IPv6 multicast addresses can be made up using the old format (RFC 2373) or the new format (RFC 3306, updated by RFC 7371).

The prefix holds the value ff for all multicast addresses.

Currently, 3 of the 4 flag bits in the flags field (ff1) are defined;^[10] the most-significant flag bit is reserved for future use. The other three flags are known as R, P and T.

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

The service is identified in the 112-bit 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.

The following table is a list notable IPv6 multicast addresses that are registered with the Internet Assigned Numbers Authority (IANA).^[16]

Ethernet

Ethernet frames with a value of 1 in the least-significant bit of the first octet^{[note 3]} of the destination 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 network equipment 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 relayed by MAC bridges conforming to 802.1D.^[17]

802.11

802.11 wireless networks use the same 01:00:5E:xx:xx:xx and 33:33:xx:xx:xx:xx MAC addresses for multicasting as Ethernet.

See also

• Broadcast address
• Reserved IP addresses

Notes

References