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In computer networking a multicast address is an identifier for a group of hosts that have joined a multicast group. Within IP it is historically known as a class D address, a type of IP address. It ranges from 224.0.0.0 to 239.255.255.255 or 224.0.0.0/4. Address assignments out of this range are specified in the IETF 'best current practice' document also known as RFC 3171.
The 224.0.0.0 to 224.0.0.255 range is assigned to multicasting on the local LAN only. Well known examples are RIP which uses 224.0.0.9 and OSPF uses 224.0.0.5
The 233.0.0.0/8 range has been assigned as public multicast address space that is aligned along autonomous system number allocations. Each byte of the AS number corresponds to the second and third octets of the multicast address.
