Abstract: Multicast communications are expanded to include the concept of private multicasts. An address space dedicated to multicast is partitioned into a subspace for public multicasts and a subspace for private multicasts. A public key/private key encryption pair is used for private multicasts and installed on domain name servers or on certification authorities. Portions of a multicast join request are sent together with a corresponding encrypted version. Private multicast equipped routers receive the multicast join request, retrieve the public key from a domain name server or from a certification authority and decrypt the encrypted portion of the join request to determine if the requester is authorized. Group specific multicast joins are also permitted by sending a bit-mask identifying a group of senders which are authorized or prohibited from sending to a user joining a multicast.