With IP, there are ranges of addresses which are designated as multicast address. A computer that wants to send out a multicast stream sets the IP destination address to a designated multicast address.
When a receiving device (computer of other type of host) want to receive a stream, it does 2 things. First it sends an IGMP join query to the nearest router. Secondly it asks its network hardware to start receiving packets from the Ethernet address corresponding to the IP multicast address of the stream.
