SIPRNet

The Secret Internet Protocol Router Network (SIPRNet) is "a system of interconnected computer networks used by the U.S. 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 '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.

SIPRNet is the SECRET component of the Defense Information Systems Network. Other components handle communications with other security needs, such as NIPRNet which is used for nonsecure communications, and JWICS which is used for Top Secret communications.

According to the U.S. Department of State Web Development Handbook, domain structure and naming conventions are the same as for the open internet, except for the addition of a second-level domain, like, e.g., "sgov" between state and gov: openforum.state.sgov.gov. Files originating from SIPRNet are marked by a header tag "SIPDIS" (SIPrnet DIStribution). A corresponding second-level domain smil.mil exists for DoD users.

According to the Pentagon, SIPRNet has approximately 4.2 million users. Access is also available to a "...small pool of trusted allies, including Australia, Canada, the United Kingdom and New Zealand..." This group (including the US) is known as the Five Eyes.

SIPRNet and NIPRNet are referred to colloquially as sipper-net and nipper-net (or simply sipper and nipper), respectively.

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Wikipedia