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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 physically isolated from insecure networks, such as the public Internet or an insecure local area network. Frequently the air gap is not completely literal, such as via the use of dedicated cryptographic devices that can tunnel packets over untrusted networks while avoiding packet rate or size variation. Even in this case, there is no ability for computers on opposite sides of the air gap to communicate.
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.
In environments where networks or devices are rated to handle different levels of classified information, the two (dis-)connected devices/networks are referred to as "low side" and "high side", low being unclassified and high referring to classified, or classified at a higher level. This is also occasionally referred to as red or high (classified) and black or low (unclassified). To move data from the high side to the low side, it is necessary to write data to a physical medium, and move it to a device on the latter network. Traditionally based on the Bell-La Padula Confidentiality Model, data can move low-to-high with minimal processes while high-to-low requires much more stringent procedures to ensure protection of the data at a higher level of classification.
The concept represents nearly the maximum protection one network can have from another (save turning the device off). It is not possible for packets or datagrams to "leap" across the air gap from one network to another, but software such as Stuxnet has been known to bridge the gap by exploiting security holes related to the handling of removable media.
The upside to this is that such a network can generally be regarded as a closed system (in terms of information, signals, and emissions security) unable to be accessed from the outside world. The downside is that transferring information (from the outside world) to be analyzed by computers on the secure network is extraordinarily labor intensive, often involving human security analysis of prospective programs or data to be entered onto air-gapped networks and possibly even human manual re-entry of the data following security analysis.
Examples of the types of networks or systems that may be air gapped include: