Cooperation between different mobile networks linked by multiple-interface mobile stations (MMSs) can yield many benefits, including coverage expansion, load balancing, and throughput improvement. We developed a network control mechanism, Cooperative Networking, which controls the cooperative benefits in heterogeneous infra-multihop networks. The proposed mechanism assigns a cooperation rule to MSs. By following the rule, every MS chooses a path to a base station, such as direct connection to 3G networks or infra-multihop connection to wireless local area networks. Cooperation rules are designed according to the cooperative benefits. In our proposed cooperative networking mechanism, network operators can adaptively select a networking policy appropriate for network conditions and the needs of users. Computer simulation results validated our proposed mechanism.