The World Wide Web (WWW) is a tool that lets individuals broadcast contents, and streaming data is rapidly increasing its percentage of the total traffic carried. Conventional broadcast techniques, however, waste too much bandwidth because the servers send streaming data directly to each recipient, even if there are many of them. This paper proposes a new IP-unicast-based multicast technology, called "Flexcast." Flexcast autonomously updates the optimal delivery tree when recipients emerge or disappear. Flexcast also maintains the optimal delivery tree in spite of user host mobility and/or changes in IP routing; the Flexcast protocol is so simple that it is extremely scalable. Its validity and feasibility are confirmed by a bench-test.