TCPlIP Internet Terms In Alphabetical Order 719 X25NET (X.25 NETwork) A service offered by CSNET that passed IP traffic between a sub- scriber site and the Internet using X.25. X.400 The ITU protocol for electronic mail. XDR (external Data Representation) The standard for a machine-independent data representation. To use XDR, a sender translates from the local machine representa- tion to the standard external representation and a receiver translates from the exter- nal representation to the local machine representation. X-Window System A software system developed at MIT for presenting and managing output on bit- mapped displays. Each window consists of a rectangular region of the display that contains textual or graphical output from one remote program. A special program called a window manager allows the user to create, move, overlap, and destroy win- dows. zero window See closed window. zone of authority Term used in the domain name system to refer to the group of names for which a given name server is an authority. Each zone must be supplied by two name servers that have no common point of failure. . [1999], Internetworking With TCP/IP: Volume 11: Design, Implementation, and Internals, 3rd edition, Prentice-Hall, Upper Saddle River, New Jersey. COMER, D. E. and D. L. STEVENS [1996], Internetworking. Prentice-Hall, Upper Saddle River, New Jersey. COMER, D. E. and D. L. STEVENS [1994], Internetworking With TCPLP Volume III - Client- Server Programming And Applications, AT&T TLI version,. Prentice-Hall, Upper Saddle River, New Jersey. COMER, D. E. and D. L. STEVENS [1997], Internetworking With TCPm Volume III - Client- Server Programming And Applications, Windows Sockets version,