The broadcast storm becomes a very serious problem for ad hoc networks to migrate into the overlay networks for the application of group communications. The main concept of virtual subnet technology is the capability to group users into broadcast domains, which divides a virtual subnet into logic, instead of physical, segments and reduces the traffic overhead. With this characteristic, we propose a novel interoperability: network model integrating a self-organizing ad hoc network and the Internet/a conventional network with the partition of physical/virtual subnets. Moreover, we describe a protocol to establish the virtual broadcast domains by using the IPv6 multicast-membership in ad hoc networks and perform IP-based network communications in a multi-switch backbone. Since the virtual subnet technology functions by logically segmenting the network into different broadcast domains, packets can only be delivered between fixed/mobile nodes with the same virtual identity. Therefore we can prevent the broadcast storm problem in MANET. The hierarchical networks, physical/virtual subnets, addressing method and mobility management are described, and some performance issues are evaluated.