SECRET Internet Protocol Router Network

Overview
The Secret Internet Protocol Router Network (SIPRNet) is

"a system of interconnected computer networks used by the United States Department of Defense and the U.S. Department of State to transmit classified information (up to and including information classified SECRET) by packet switching over the TCP/IP protocols in a "completely secure" environment."

It also provides services such as hypertext document access and electronic mail. As such, SIPRNet is the DoD's classified version of the civilian Internet.