MILNET was the name given to the part of ARPANET, the forerunner of the Internet, that was designated for nonclassified U.S. military use. When MILNET was split off from ARPANET in 1983, ARPANET remained for use by the academic research community. However, gateway computers interconnected the networks.

Today, the top level domain (TLD) name of .mil is reserved for use only by U.S. military agencies.

It was used for unclassified United States Department of Defense (DoD) 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.

In 1983 ARPANET split into ARPANET and MILNET, removing the military component from ARPANET.  MILNET, designed for unclassified U.S. Department of Defense traffic, was integrated into the Defense Data NetworkOffsite Link that had been created the previous year.

MILNET was physically separated from the ARPANET in 1983. The ARPANET remained in service for the academic research community, but direct connectivity between the networks was severed for security reasons. Gateways relayed electronic mail between the two networks. BBN Technologies built and managed both the MILNET and the ARPANET and the two networks used very similar technology. It is also known as “Military Net.”

During the 1980s the MILNET expanded to become the Defense Data Network, a worldwide set of military networks running at different security levels. In the 1990s, MILNET became the NIPRNET.