We propose a routing overlay system, MobCast, for simple and efficient routing to mobile hosts. Mobcast nodes advertise the same address space at each proxy location, and each mobile host is assigned a "universal" IP address from this address space, so packets sent to a mobile host's universal IP address automatically go to the nearest proxy on the overlay. The overlay then delivers the packets to the mobile host. Our architecture enables seamless mobility for both micro and macro mobility. While our initial design is not as mature as Mobile IP, it shows great promise to solve the traditional problems of ingress routing, firewalls, NATs, and rapid mobility with much lower complexity. We present our design as a scalable and deployable alternative to Mobile IP. In this paper, we focus on describing the MobCast system architecture. We form our arguments for scalability, handoff-speed, and simplicity, and give our initial results for scalability. We postpone a detailed discussion of MobCast's security model for future work.