Recent technological innovations allow mobile devices to be equipped with multiple wireless interfaces. Moreover, the trend in fourth-generation or next-generation wireless networks (4G/NGWNs) is the coexistence of diverse but complementary architectures and wireless access technologies. In this context, an appropriate mobility management scheme, as well as the integration and interworking of existing wireless systems, is crucial. Several proposals for solving these issues are available in the literature. However, these proposals cannot guarantee seamless roaming and service continuity. This paper proposes a novel architecture called Integrated InterSystem Architecture (IISA), which is based on the Third-Generation Partnership Project/Third-Generation Partnership Project 2 requirements that enables the integration and interworking of current wireless systems, and investigates mobility management issues. An efficient handoff protocol based on localized mobility management, access networks discovery, and fast handoff concepts called Handoff Protocol for Integrated Networks (HPINs) is proposed. It alleviates service disruption during handoff in IPv6-based heterogeneous wireless environments. Numerical results show that HPIN performs better in terms of signaling cost, handoff latency, handoff-blocking probability, and packet loss compared to existing schemes.