Network Terms "X"

X.400 standard

The X.400 standard contains guidelines for exchanging messages electronically. It includes rules for addressing messages as well as an addressing scheme that uses management domains, originator/recipient names, and other information to uniquely identify users within the hierarchy of the organization.

X.500

X.500 is a set of recommendations from the International Telecommunications Union. It lays out the rules that allow combining local directory services into one global directory.