We propose a new forwarding mechanism for IP multicast based on explicit multicast (Xcast). Compared with traditional IP multicast, our mechanism achieves a more flexible allocation of multicast forwarding states among routers. Previous works only focus on reducing the number of routers which store the forwarding states, ignoring the distribution of forwarding states among routers. The number of forwarding states stored in each router may be unbalanced. We formulate the assignment of the routers with forwarding states as two optimization problems. The first one is to minimize the number of routers with the forwarding states in each multicast tree. The second one is to minimize the maximum number of forwarding states stored in a router. We design several algorithms for both problems. We prove that our algorithms can find the optimal solution to the first problem. By simulation, we also show that our algorithms perform well in the second problem. Moreover, we prove that assigning all branching routers as the only routers with the forwarding states is a special case in our mechanism. Due to higher flexibility, we show that our mechanism use fewer forwarding states, and the distribution of forwarding states is more balanced.