Abstract: First introduced in the paper here is the concept of IP multicast and D class IP addresses. It is shown how an IP workstation can join and leave a multicast group, and maintain its membership in the multicast group. Routing in multicast is also different from that in unicast. The paper gives the forms of two different routing trees, and shows the differences of these trees. Then it shows the scope of IP multicast. Securities in multicast are discussed in detail. The paper also describes how the security keys are managed, how a workstation gets a key for secure multicast, and how the key is used in data transmission. Reliable data transmission in multicast is discussed too. The second part of the paper gives an example of using IP multicast\|IP TV. The architecture of IP TV is outlined, four kinds of workstations in IP TV are shown, and then its quality of service is discussed.