An air gap is a security measure implemented for computers, computer systems or networks. It ensures total isolation of a given system – electromagnetically, electronically, and, most importantly physically – from other networks, especially those that are not secure. The usual configuration of an air gap is a sneakernet (data is typically moved between computers on foot), in which alternate storage, like flash drives or CDs, must be used to transfer data to and from the isolated device, rather than simply moving data across shared drives and networks.