what is X Series for Network Communications

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