Abstract: Disclosed are a network system which can ensure the security in a LAN environment, an authentication method and a program used therein. A switching hub attains an authentication frame transmitted from a terminal and copies the frame content to use it as an authentication packet for making an inquiry about the authentication of the terminal to an authentication server. The authentication server then retrieves to check whether or not the MAC address included in the authentication packet is stored in an authentication database. In the case where an authentication method is a password, when the password in the authentication packet is correct, the authentication server returns the authentication packet (OK) to the switching hub and, when the MAC address is not stored in the authentication database or the password is incorrect, returns an authentication packet (NG) notifying that the terminal is used by a false user.