DSN stands for defense switched network, which is the primary system of interbase communication for the U.S. Department of Defense. A DSN number is a phone or data communications number that is used within that closed system.
The DSN is the primary means of information transfer for the Defense Information System Network. Its main function is to provide worldwide voice, facsimile, data and video teleconferencing services for Department of Defense command and control. It assures continual communications services during peacetime as well as during crisis and wartime. The system provides interconnection of all worldwide military locations and is designated the long-distance provider of the DoD.