A gateway device in an IP communications system can be controlled by a gateway controller even when an IP address of the gateway device is not fixed. The IP communications system includes a gateway device having a subscriber terminal interconnected thereto, a gateway controller for controlling the gateway device, provided on a subscriber side in accordance with a protocol conforming to the master-slave architecture, a database for providing subscriber information to the gateway controller, and an IP address allocator for dynamically allocating IP addresses. The gateway device requests the IP address allocator to allocate an IP address to acquire an IP address, and notifies the gateway controller of that IP address to get the IP address stored in the database.