A set of recommendations adopted by the International
Telecommunication Union Telecommunication Standardization Sector ITU-T,
formerly the CCITT, and International Organization for Standardization ISO for
standardizing equipment and protocols used in both public access and private
computer networks.
| Recommendation Number |
What It Covers |
| X.25 |
Interface required to connect a computer to a packet-switched network such
as the Internet |
| X.75 |
Protocols for connecting two public data networks |
| X.200 |
Seven-layer set of protocols known as the ISO/OSI reference model for
standardizing computer-to-computer connections |
| X.400 |
Format at the ISO/OSI application layer for e-mail messages over various
network transports, including Ethernet, X.25, and TCP/IP. Gateways must be used
to translate e-mail messages between the X.400 and Internet formats |
| X.445 |
Asynchronous Protocol Specification, which governs the transmission of X.400
messages over dial-up telephone lines |
| X.500 |
Protocols for client/server systems that maintain and access directories of
users and resources in X.400 form |
| X.509 |
Digital certificates |