A network address conversion system for enabling access to a specific node of a private network, having a private IP address, a method therefor, and a recording medium for recording the method. The network address conversion system includes a reservation unit for receiving an access reservation demand from an external network node to access a specific node of the private network, an external port value allocation unit for allocating the external port value to the specific node, and transmitting the external port value to the external network node, a mapping table for storing the external port value, and an address conversion unit for converting the external port value into a private IP address of the specific node, when the external network node accesses the specific node by using the external port value. As a result, the external network node of the Internet can access the specific node of the private network, having the private IP address.