This figure represent a simplified nuclear submarine having 3 isolated networks (red, green and blue). In reality there are many more internal networks with different security levels for utilities, weapon systems, life support systems, video, VoIP etc. Networks with the same function and same security level are also segmented to improve reliability and security. The set of requirements for equipment that touches these networks is very challenging. System must be secure to avoid leakages between networks and external intrusions. System also must recover in fast and controllable manner from catastrophic events (surviveability). Submarine systems must be designed for lowest possible eletromagnetic emmisions (TEMPEST).