Overview of IP Multicasting

In the RFC 1112, IP multicasting is defined as the transmission of an IP datagram to a group of hosts identified by a single IP destination address.

In addition to this common multicast group address, the hosts in the group all have separate and unique unicast addresses. The actual multicast host group can consist of any number of hosts, possibly even located in different networks. The number can vary over time, as hosts can join in and leave from a group at any time. Moreover, a particular host can belong to several groups simultaneously.

The multicast group addresses are class D addresses. They are identified by the high-order initial four-bit sequence 1110. In the dotted decimal notation, the multicast group address range runs from 224.0.0.0 to 239.255.255.255. There are certain special addresses:
  • 224.0.0.0 is never assigned.
  • 224.0.0.1 is assigned to the permanent group of all hosts, including gateways, in the local network.
  • 224.0.0.2 is assigned to all local multicast routers.

Multicast IP addresses are not allowed to be used as source addresses. A multicast source address implies forging of an IP address.

The multicast groups are either permanent or transient. Permanent groups have administratively assigned IP addresses, while the addresses of the transient multicast groups can be assigned dynamically from the pool of multicast addresses not reserved for permanent groups. The IP address of an established permanent group persists even if the group would not have any members at a given time. The transient groups cease to exist as soon as they no longer have member hosts, and the assigned multicast address is released.

See, for example, http://⁠www.iana.org/assignments/multicast-addresses for a list of addresses registered with IANA.