Multihoming is an attractive feature for mobile nodes (MN) equipped with multiple network interfaces in a converged networking environment. However, the current standard Mobile IP (MIP) lacks support for multihomed MNs. Accordingly, this paper presents an immediate solution to resolve the problems of multihomed MNs in a UMTS and WLAN interworking system. The interworking system is based on a hierarchical architecture according to the UMTS and WLAN coverage characteristics. The MIP is only operated in the WLANs, and not in the UMTS, allowing the MIP to be used for multihomed MNs with minimal modifications. In addition, MIP binding filters are used to realize the advantages of multihoming, enabling different traffic flows to be transmitted simultaneously through different interfaces. The overall design aims to minimize the modification to the MIP and impact on existing UMTS and WLAN systems. We evaluate the performance of proposed scheme using an OPNET simulator.