g is similar to a pay-per-view cable television channel. A viewer can tune if they are authorized to do SO. To handle the invitation process, a sending computer specifies a group of recipients by transmitting a single copy of its multicast to a special IP address. Special protocols are employed to translate from the group address to the IP addresses or group members, and from there, to the MAC addresses of individual hosts. Multicasting also requires a special protocol that is used to manage group membership. Lastly, it requires routers that can interpret multicast protocols and use them to distribute packets only to the proper networks. These three special multicast-specific functions-address translation, dynamic management of group membership, and multicast-aware routing-are discussed in the next section. Layer-3-to-Layer-2 Address Translation The hosts that combine to make up a multicast group are assembled under a special Video Communications: The Whole Picture class of IP address. This address category is known as a Class D address. There are also Class A, B and C addresses (used for unicasts and broadcasts) and Class E addresses (unassigned at present but reserved for the future). Class D addresses exist at the network-layer and are used exclusively for multicast traffic. These addresses are allocated dynamically; each represents a select group of hosts interested in receiving a specific multicast. The network-layer (Class D IP address) that is used to communicate with a group of receivers must be mapped over to the data-link layer (typically Ethernet MAC) addresses associated with each group member. Translation between an IP multicast address and a data-link layer address is accomplished by dropping the lowest-order 23 bits of the IP address into the low- order 23 bits of the MAC address. In order for a host to participate in a multicast, its TCP/IP stack must be IP-multicast aware- most, but not all, are. Network adapters must be multicast-enabled, too. Good ones allow several MAC addresses to be programm