Sets of data telecommunications protocols and interfaces defined by CCITT Recommendations. Note: Some of the more common X.-series Recommendations are:
X.25: A CCITT Recommendation for public packet switched communications between a network user and the network itself.
X.75: A CCITT 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.