A circuit level gateway is a type of firewall that operates at the session layer of the OSI model, which is Layer 5. This kind of firewall is designed to provide security by validating and managing sessions without inspecting the actual contents of each packet. It is particularly adept at hiding the private network information because it only allows traffic through that is part of an established session, effectively masking the details of the network's internal structure from the outside. This makes it an ideal choice for John's requirements.