The present invention includes a method for preventing any external intruder by filtering an Internet Protocol packet in customer premise equipment connected between a personal computer (PC) and a digital subscriber line (xDSL) when it is operated in a bridge mode. To this end, the present invention provides the method including the steps of: detecting a destination IP address by analyzing the IP packet to be transmitted to the other party as a PC user access to an Internet through xDSL; registering the destination IP address to a predetermined allowance table; detecting a source IP address of the IP packet upon receiving the IP packet through xDSL; and checking whether the source IP address is duly registered to the allowance table for passing the corresponding IP packet through the PC only if the source IP address has been registered, and for blocking the corresponding IP packet if the source IP address has not been registered.