A multicast address is a logical identifier for a group of hosts in a

computer network A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are ...

that are available to process

datagram A datagram is a basic transfer unit associated with a packet-switched network. Datagrams are typically structured in header and payload sections. Datagrams provide a connectionless communication service across a packet-switched network. The del ...

s or frames intended to be

multicast In computer networking, multicast is group communication where data transmission is addressed to a group of destination computers simultaneously. Multicast can be one-to-many or many-to-many distribution. Multicast should not be confused with ...

for a designated network service. Multicast addressing can be used in the link layer (layer 2 in the

OSI model The Open Systems Interconnection model (OSI model) is a conceptual model that 'provides a common basis for the coordination of SOstandards development for the purpose of systems interconnection'. In the OSI reference model, the communications ...

), such as

Ethernet Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 1 ...

multicast, and at the

internet layer The internet layer is a group of internetworking methods, protocols, and specifications in the Internet protocol suite that are used to transport network packets from the originating host across network boundaries; if necessary, to the destinat ...

(layer 3 for OSI) for

Internet Protocol The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet. ...

Version 4 (IPv4) or Version 6 (IPv6) multicast.

IPv4

IPv4 multicast addresses are defined by the

most-significant bit In computing, bit numbering is the convention used to identify the bit positions in a binary number. Bit significance and indexing In computing, the least significant bit (LSB) is the bit position in a binary integer representing the binar ...

pattern of ''1110''. This originates from the

classful network A classful network is an obsolete network addressing architecture used in the Internet from 1981 until the introduction of Classless Inter-Domain Routing (CIDR) in 1993. The method divides the IP address space for Internet Protocol version 4 (IP ...

design of the early Internet when this group of addresses was designated as ''Class D''. The CIDR notation for this group is . The group includes the addresses from to . Address assignments from within this range are specified in , an

Internet Engineering Task Force The Internet Engineering Task Force (IETF) is a standards organization for the Internet and is responsible for the technical standards that make up the Internet protocol suite (TCP/IP). It has no formal membership roster or requirements and a ...

(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 to are individually assigned by IANA and designated for multicasting on the local subnetwork only. For example, the

Routing Information Protocol The Routing Information Protocol (RIP) is one of the oldest distance-vector routing protocols which employs the hop count as a routing metric. RIP prevents routing loops by implementing a limit on the number of hops allowed in a path from sour ...

(RIPv2) uses ,

Open Shortest Path First Open Shortest Path First (OSPF) is a routing protocol for Internet Protocol (IP) networks. It uses a link state routing (LSR) algorithm and falls into the group of interior gateway protocols (IGPs), operating within a single autonomous syst ...

(OSPF) uses and , and Multicast DNS uses . Routers must not forward these messages outside the subnet from which they originate. ;Internetwork control block :Addresses in the range to 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 . ;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. ;Source-specific multicast :The (IPv4) and (IPv6) blocks are reserved for use by source-specific multicast. ;GLOP :The range was originally assigned by 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

octet Octet may refer to: Music * Octet (music), ensemble consisting of eight instruments or voices, or composition written for such an ensemble ** String octet, a piece of music written for eight string instruments *** Octet (Mendelssohn), 1825 compos ...

s 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 ASNs. , superseding , 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. ;Unicast-prefix-based :The range is assigned by as a range of global IPv4 multicast address space provided to each organization that has or larger globally routed unicast address space allocated; one multicast address is reserved per of unicast space. A resulting advantage over GLOP is that the unicast-prefix mechanism resembles the unicast-prefix capabilities of IPv6 as defined in . ;Administratively scoped :The range is assigned by 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 range to be loosely similar to the scoped IPv6 multicast address range described in .

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 The Internet Assigned Numbers Authority (IANA) is a standards organization that oversees global IP address allocation, autonomous system number allocation, root zone management in the Domain Name System (DNS), media types, and other Inte ...

(IANA).

IPv6

Multicast addresses in

IPv6 Internet Protocol version 6 (IPv6) is the most recent version of the Internet Protocol (IP), the communications protocol that provides an identification and location system for computers on networks and routes traffic across the Internet. IPv ...

use the prefix . IPv6 multicast addresses can be structured using the old format () or the new format (, updated by ). The ''prefix'' holds the value 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. 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 ''group ID'' field. For example, if refers to all Network Time Protocol (NTP) servers on the local network segment, then 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.

Ethernet

Ethernet frames with a value of 1 in the least-significant bit of the first octetOn Ethernet, the least-significant bit of an octet is the first to be transmitted. A multicast is indicated by the first transmitted bit of the destination address being 1. of the destination

MAC address A media access control address (MAC address) is a unique identifier assigned to a network interface controller (NIC) for use as a network address in communications within a network segment. This use is common in most IEEE 802 networking tec ...

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 () are sometimes referred to as

broadcasts Broadcasting is the distribution of audio or video content to a dispersed audience via any electronic mass communications medium, but typically one using the electromagnetic spectrum (radio waves), in a one-to-many model. Broadcasting began wi ...

, 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 to for group addresses for use by standard protocols. Of these, the MAC group addresses in the range of to are not forwarded by

802.1D IEEE 802.1D is the Ethernet MAC bridges standard which includes bridging, Spanning Tree Protocol and others. It is standardized by the IEEE 802.1 working group. It includes details specific to linking many of the other 802 projects including t ...

-conformant

MAC bridge A network bridge is a computer networking device that creates a single, aggregate network from multiple communication networks or network segments. This function is called network bridging. Bridging is distinct from routing. Routing allows mu ...

802.11

802.11 IEEE 802.11 is part of the IEEE 802 set of local area network (LAN) technical standards, and specifies the set of media access control (MAC) and physical layer (PHY) protocols for implementing wireless local area network (WLAN) computer com ...

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

Notes

References

{{DEFAULTSORT:Multicast Address Network addressing