Defense Data Network

The Defense Data Network (DDN) was a computer networking effort of the United States Department of Defense from 1983 through 1995.

History
In 1975, the Defense Communication Agency (DCA) took over operation of the ARPANET as it became an operational tool instead of a research project. In 1983, plans for a new generation of the Automatic Digital Network (Autodin II) were canceled. Instead, a separate network to connect military installations called MILNET was split off the ARPANET. The ARPANET would be used as an Internet  backbone for researchers, but be slowly phased out. Both networks carried unclassified information, and were connected at a small number of points which would allow total separation in the event of an emergency. The DCA used the Defense Data Network (DDN) as the program name for these network programs.

As a large-scale, private internet, the DDN provided Internet Protocol connectivity across the United States and to US military bases abroad. Throughout the 1980s it expanded as a set of four parallel military networks, each at a different security level. These networks transitioned to become the NIPRNET, SIPRNET, and JWICS networks in the 1990s.

The four DDN subnetworks were:


 * Military Network (MILNET) for Unclassified traffic
 * Defense Secure Network One (DSNET 1) for Secret traffic
 * Defense Secure Network Two (DSNET 2) for Top Secret traffic
 * Defense Secure Network Three (DSNET 3) for Top Secret/Sensitive Compartmented Information (TS/SCI)

MILNET and DSNET 1 were common user networks, much like the public Internet, but DSNET 2 was dedicated to supporting the Worldwide Military Command and Control System (WWMCCS) and DSNET 3 was dedicated to supporting the DOD Intelligence Information System (DODIIS).

DDN-NIC
DDN-NIC or Network Information Center (NIC) was located at the DDN Installation and Integration Support (DIIS) program office in Chantilly, Virginia. It provided general reference services to DDN users via telephone, electronic mail, and U.S. mail. It was the first organization responsible for the assignment of TCP/IP addresses and Autonomous System numbers.