The MILNET, also called the Military Network or Military Net, was part of the ARPANET, which was the original name of the Internet. It was used for unclassified United States DoD (Department of Defense) traffic. MILNET was split from the ARPANET in 1983, leaving the ARPANET to be used for academic research and public use. All links between MILNET and ARPANET were severed when MILNET split off. E-mail between the two networks were delivered using gateways.
BBN Technologies built and maintained MILNET and ARPANET. In the 1980s, MILNET was expanded and became the Defense Data Network. This large network was a compilation of multiple smaller military networks, each one running at a different security level. In the 1990s, MILNET gave birth to NIPRNet, which was used to transmit sensitive, but unclassified data between internal users and also provide those users with Internet access.