X.-series Recommendations

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.