Sets of data telecommunications protocols and interfaces defined by CCITT (now, ITU-T) Recommendations. Note: Some of the more common X. -series Recommendations are:X. 25: A Recommendation for public packet switched communications between a network user and the network itself. X. 75: A Recommendation for public packet switched communications between network hubs. X. 400: An addressing scheme for use with e-mail. X. 500: An addressing scheme for directory services.