This setup is best known as an air gap. In network an air gap means two or more networks are physically separated from each other to ensure no data can traverse from one to the other. Generally if a network is so critical it requires an air gap it will be a completely stand alone network with no access to other networks and especially the internet. A true air gap is not common in most businesses, but some known examples are government or military networks, highly critical infrastructure networks like nuclear power plant controls and financial systems like stock exchanges.