ddn

from The Free On-line Dictionary of Computing (8 July 2008)

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)