The Free On-line Dictionary of Computing (30 December 2018):

Defense Data Network
DDN

   (DDN) A global communications network serving the US
   Department of Defense.  Composed of MILNET, other portions
   of the Internet, and classified networks which are not part
   of the Internet.  The DDN is used to connect military
   installations and is managed by the Defense Information
   Systems Agency.

   (1994-12-01)