A series of standards established by the CCITT covering the communications between digital equipment, such as computers and modems, and digital networks. The X.25 standard addresses the interface between a computer or data terminal and a digital network and is the most common standard in the series. The X.21 standard addresses the interface to modems. Other standards within the series address how blocks of data are formatted, how public data networks should be numbered, and the open systems interconnection (OSI) model.