1. A method for summarizing a default address of private network-network interface lowest level node, which is applicable to an asynchronous transfer mode (ATM) switching system employing private network-network interface (PNNI) for public networks, comprising: a first step of constructing a database for storing prefix information, node information, and reachable address information; a second step of arranging parameters inputted for creating an internal prefix to be adaptable to the system, generating the prefix information by checking the validation and address system of the parameters inputted for creating the internal prefix, and registering the prefix information in the prefix information database; a third step of creating an ATM end system address by checking the validation and address system of parameters inputted for generating the node information, and if it is determined that the ATM end system address includes internal prefix and a node address is not yet assigned from the digit state of the ATM end system address, generating the node information using the parameters for creating the node information and the ATM end system address, and registering the node information in the node information database; and a fourth step of summarizing the node address of the node information up to the bit length of an internal prefix and performing summarization of a default reachable address by defining the internal prefix as the default reachable summarization address.