User authentication system and user authentication method

A subscriber's terminal is configured to be connected to an internet service provided via a LAN and an IP subnet distribution switch within a network service provider When a packet signal that has not received the authentication is input, a physical-port changeover switch gives a temporary IP address to the subscriber's terminal and causes the subscriber's terminal to employ this for making an authentication process. If the authentication succeeds, a normal IP address is given, and the packet signal, which made use of this, is distributed to a network that is an object by the IP subnet distribution switch