A method of processing data in a network printing system having a network printer and at least one host computer connected thereto, the method including adding an address list of a host computer that is designated as allowed to print through the network printer to predetermined document data according to a selection of a user, determining whether an address, which is identical to that of the host computer, exists in the address list when a command to print the document data is input, and printing the document data if the address, which is identical to that of the host computer, is determined to exist in the address list.