MilNet, which is a short form for Military Network, was the name given to Advanced Research Projects Agency Network (ARPANET). It carries only unclassified information and was meant to use for United States Department of Defense traffic. In 1983, both ARPANET and MILNET split and divided into two networks.
Henceforth, it was decided that ARPANET would dedicate itself completely to support academic research community and MILNET would be used to transmit U.S. military data, thereby serving the needs of the military. And the direct connectivity between the two networks was cut off for some security reasons. These two networks were built and managed by BBN Technologies and hence, they used the same technology.
The new standard of rules and the split made it believe that ARPANET was originally created for military purposes. The development of Transmission Control Protocol (TCP) and Internet Protocol (IP) and its decentralized structure helped ARPANET to develop considerably in the eighties. On the other hand, MILNET, which took possession of 65 nodes or points of connection out of 113 to safeguard military data, expanded further that it was contained by the Defense Data Network (DDN). DDN is a worldwide set of military networks that are used at different security levels.
In 1990, the scenario changed. The world realized the potential for massive interconnection and it led to the advent of the Internet. The military agencies which understood this potentiality went for the further expansion of MILNET. It started providing Internet Protocol (IP) connectivity to various United States military bases both at home and abroad.
Then it was split into several networks like the Secret Internet Protocol Router Network (SIPRNET), Non-Classified Internet Protocol Router Network (NIPRNET) and the Joint Worldwide Intelligence Communications System (JWICS). Today a section of MILNET that became NIPRNET is used by the United States military to exchange sensitive and unclassified data among the internal users.