An air gap or air wall is a security measure often taken for computers and computer networks that must be extraordinarily secure. It consists of ensuring that a secure network is completely physically, electrically, and electromagnetically isolated from insecure networks, such as the public Internet or an insecure local area network. Limitations imposed on devices used in these environments may include a ban on wireless connections to or from the secure network or similar restrictions on EM leakage from the secure network through the use of TEMPEST or a faraday cage. It is most recognizable in the time-honored configuration known as "sneaker-net" where the only connection between two devices or networks is via a human being providing media-switching, i.e.; floppies, CDs, or USB drives. The term derives from the notion that one must put on sneakers and walk to transfer data.