The Defense Data Network (DDN) is the global data network of the U.S. Department of Defense, the Department of Defense (DoD). This global communications and data network is part of the ARPANET
and was used by the U.S. Department of Defense for their Internet services from 1983 to 1996. The Defense Data Network evolved from MILNET and uses the IP protocol as its network protocol
. During its use, the DDN network was divided into several subnets with different security levels: Military Network (MILNET) for non-qualified traffic, Defense Secure Network One (DSNET 1) for security-relevant traffic, Defense Secure Network Two (DSNET 2) for top-secret traffic, and Defense Secure Network Three (DSNET 3) for sensitive top-secret traffic.