sentences:  In 2013 the working, Industrial Control System Cybersecurity, directed by the French Network and Information Security Agency (ANSSI) stated that is forbidden to use firewalls to connect any class 3 network, such as railway switching systems, to a lower class network or corporate network, only unidirectional technology is permitted.Unidirectional network devices are typically used to guarantee information security or protection of critical digital systems, such as Industrial control systems, from cyber attacks.While use of these devices is common in high security environments such as defense, where they serve as connections between two or more networks of differing security classifications, the technology is also being used to enforce one-way communications outbound from critical digital systems to untrusted networks connected to the Internet.