air gap - Air Gap - AIR GAP
Any industrial control network should NEVER be connected to the internet unless it is unavoidable no matter what any idiotic senior managers want.
Most industrial control systems were designed in the days when their security depended on there being no access outside the plant being controlled. This resulted in the majority of the controllers having virtually no internal security measures - the assumed air gap was their security.
(If there is a need for control over the internet (eg for an unmanned pumping station) then the communication link should be via a firewall at the remote unit that only allows a few (preferably one) PC to exercise the control.)
Icon for the people who blindly connect industrial control systems to the internet ==========>