By keeping routers informed about multicast hosts, it allows multicast datagrams to traverse an internetwork and reach many hosts simultaneously. A special router encapsulates multicast datagrams and sends them as unicast IP datagrams to other special routers, which de-encapsulate them and send them on the local network as standard multicast datagrams. The video was choppy and the audio low-fidelity, but like early radio and TV broadcasts it got everyone excited. It was a harbinger of what's to come. The address 224.0.0.0 is reserved, and 224.0.0.1 is assigned to the permanent group of all IP hosts.
Any host can send a multicast datagrams without being a group member.
Multicast addresses provide a limited broadcasting, but without the problems and limitations of traditional broadcasts. As a result, multicast datagrams don't incur the extra overhead that traditional broadcast packets do. Sending datagrams to a multicast address is analogous to transmitting radio signals on a particular frequency. Just as you must tune a radio receiver to the particular frequency to receive the radio signal, you must join a multicast group to receive the multicast packets. Unlike a radio transmission, however, you need not be in range to receive the signal.