Summary
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. The group includes the addresses from 224.0.0.0 to 239.255.255.255. Address assignments from within this range are specified in , 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. Source-specific multicast The 232.0.0.0 (IPv4) and FF3x:: (IPv6) blocks are reserved for use by source-specific multicast.
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