An authenticating unit authenticates an external terminal and stores the result of authentication in an authentication state table. A receiving unit receives a first message containing information relating to a first application and identification information unique to the external terminal. A determining unit determines whether the external terminal contained in the first message is authentic by referring to the information in the authentication state table, each time the first message is received. A generating unit generates a second message containing a port, which is to be used by the first application, and an address of the external terminal when the external terminal is determined to be authentic. A transmitting unit transmits the second message to a firewall.