A PPPoA (point-to-point (PPP) over asynchronous transfer mode (ATM)) spoofing function utilizing an asymmetric digital subscriber line (ADSL) modem to form a single network between a client PC and a network access server (NAS) by allowing the ADSL modem to make a PPP connection to the NAS when the client PC is booted, by allowing the NAS to transmit Internet protocol (IP) configuration information, including a global IP address, to a DHCP server of the ADSL modem through a PPP Internet Protocol control protocol (IPCP), and by allowing the ADSL to transfer the IP configuration information received from the NAS to the client PC, thereby forming a bridge by the ADSL modem between the client PC and the NAS to enable IP packets to be transferred between the client PC and the NAS.