It is provided an authentication method for realizing a network authentication function for an authentication system, the authentication system including an authentication server for authenticating a terminal used by a user, and a switch for mediating an authentication sequence between the terminal and the authentication server. The authentication method includes steps of: providing, by the switch, identification information for identifying the switch to the authentication server in the authentication sequence; authenticating, by the authentication server, an authentication request transmitted from the terminal; transmitting, by the authentication server, an authentication result of the authentication to the switch based on the provided identification information on the switch; and authenticating, by the switch, access from the terminal based on the authentication result received from the authentication server.