With the availability and penetration of multicast-capable routers, many local networks in today's Internet are multicast-capable. However, achieving global IP multicast is still hindered by many management and technical difficulties. This is because routers interconnecting these local multicast-capable networks, or so-called "islands," are often either multicast-incapable or multicast-disabled. Traditionally, application-level multicast (ALM) has been proposed to overcome the problem. However, these protocols only make use of unicast connections to form delivery trees and have not fully taken advantage of the local multicast capability of an island. As a result, these protocols are not very efficient. In order to achieve efficient global multicast, we propose and study Island Multicast (IM) where unicast connections are used between islands while IP multicast is used within islands. We present the detailed mechanisms for IM for both distributed and centralized approaches. While the distributed approach is scalable to many users and applicable to single-source applications, the centralized one is much simpler to implement and applicable to many-to-many applications. Using Internet-like topologies, we show that the distributed approach achieves lower latency and the centralized approach has higher bandwidth efficiency. IM significantly improves network performance (in terms of stress, delay and nodal degrees) as compared to using ALM alone.