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 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 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
126.96.36.199/4. The group includes the addresses from 188.8.131.52 to 184.108.40.206. Address assignments from within this range are specified in RFC 5771, an Internet Engineering Task Force (IETF) Best Current Practice document (BCP 51).
The following table is a partial list of well-known IPv4 addresses that are reserved for IP multicasting and that are registered with the Internet Assigned Numbers Authority (IANA).
The address block 220.127.116.11/24 (18.104.22.168 to 22.214.171.124) is designated for multicasting on the local subnetwork only. For example, the Routing Information Protocol (RIPv2) uses 126.96.36.199, Open Shortest Path First (OSPF) uses 188.8.131.52, and Zeroconf mDNS uses 184.108.40.206.
The 220.127.116.11/8 range was originally assigned by RFC 2770 as an experimental, public statically assigned multicast address space for publishers and Internet service providers, that wish to source content in 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, allowing any operator assigned an ASN 256 globally unique multicast group addresses per ASN. The method is not applicable to the newer extension AS numbers which consist of 32 bits. 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.
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