The current IP multicast architecture does not have enough scalability and is not suitable for the whole Internet, because routing information is too large. We propose the scalable interdomain IP multicast architecture in order to achieve IP multicast communications over the whole Internet. The proposed architecture provides methods to reduce the routing information. We define two kind of multicast addresses, which are called VMA (virtual multicast address) and MAR (multicast address for routing). The VMA is used to identify the multicast group in the intra-domain and MAR is used to forward multicast packets between the interdomain. With the definition of two multicast addresses, multicast address allocation can be more flexible, and routing information can be reduced with address aggregation. We estimate the amount of reduced routing information with our architecture, and evaluate the proposed architecture.