In an address management system, an interface ID can be handed over from one apparatus to another without losing the correspondence between the interface ID and a user. As a result, the interface ID can be used as a unique identifier for the user. Accordingly, an effective infrastructure for individualized services is achieved. The interface ID may be constant for one user even when an apparatus is replaced, and thus IPv6 communication can be used as a user-associated address in high availability services, such as telephone services. Furthermore, the interface ID can be returned and reused, and thus the interface ID can be efficiently used.