John wants to implement a firewall service that works at the session layer of the...

 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.

References: The information about circuit level gateways operating at the session layer and their ability to hide private network information is supported by multiple sources within the field, including educational resources and security-focused articles123. Additionally, the ECCouncil’s Certified Network Defender (CND) program covers the necessary knowledge regarding network security and defense strategies, which includes understanding the functions and applications of different types of firewalls45.