The present invention relates to a system and method for authenticating, monitoring, and managing all terminals connected to a wireless/wired network to use Internet. A shared terminal management system comprises a management server, a charging server, a central server, a central authentication G/W server, and a proxy server, and assigns a terminal identification value for every terminal that uses Internet, authenticates terminals by reading and analyzing the assigned terminal identification value, monitors and manages shared terminals used as being connected to one line to classify lines into a basic line and an additional line, and charges for the additional line. The shared terminal identification system for identifying and managing terminals connected to one Internet line comprises a subscriber line authentication unit, a packet collecting unit, a first packet analyzing unit, an element packet transmission unit, a data management unit and a terminal determining unit.