In Source-Specific Multicast (SSM), receivers are aware of sources. A receiver explicitly requests a stream from a sender in a group the receiver has joined, using a Join message that specifies the source (an (S, G) Join), and explicitly excludes the use of the wildcard for source—that is, it disallows the (*, G) Join.
Because SSM-enabled hosts track sources of the multicast transmission themselves, multicast routers do not need to discover the multicast sources themselves, and they need to store and maintain only the (*, G) state in the multicast routing table. This greatly reduces the burden on the multicast router.
For IPv4, the destination addresses must be in the range 188.8.131.52/8. For IPv6, the addresses must be in the range FF3x::/96.
SSM is specified in RFC 3569, An Overview of Source-Specific Multicast (SSM) and RFC 4607, Source-Specific Multicast for IP.