Number portability is a telecommunication network function, which allows subscribers to switch services, service providers, or locations without changing their telephone numbers. Near-term number portability is typically implemented by existing telecommunication services such as remote call forwarding. Long-term number portability can be implemented under the advanced intelligent network (AIN) platform. This article focuses on the routing issues for number portability. We introduce the number portability issues and provide a tutorial on the AIN. We then show the AIN implementation for long-term number portability. Finally, a cache approach is proposed to speed up address transfer, which can effectively reduce the network overhead incurred by AIN query for number portability.