An address allocated to a user by an authentication server is used as an IP address of a packet which is transmitted from a user terminal, preventing an illicit use if the IP address were eavesdropped. An authentication server 100 performs an authentication of a user based on a user authentication information which is transmitted from the user terminal, and upon a successful authentication, allocates an address to the user terminal, and issues a ticket containing the address to be returned to the user terminal. The user terminal sets up the address contained in the ticket as a source address, and transmits the ticket to the application server 300, requesting a session to be established. After verifying that the ticket is authentic, the server 300 stores the ticket and establishes a session with the user terminal. The user terminal transmits a service request packet containing the source address to the server 300 utilizing the session. If the source address coincides with the address contained in the stored ticket, the server 300 provides a service to the user.