Can you explain circuit level gateway?

The circuit level gateway firewalls work at the session layer of the OSI model. They monitor TCP handshaking between the packets to determine if a requested session is legitimate. And the information passed through a circuit level gateway, to the internet, appears to have come from the circuit level gateway. So, there is no way for a remote computer or a host to determine the internal private ip addresses of an organization, for example. This technique is also called Network Address Translation where the private IP addresses originating from the different clients inside the network are all mapped to the public IP address available through the internet service provider and then sent to the outside world (Internet). This way, the packets are tagged with only the Public IP address (Firewall level) and the internal private IP addresses are not exposed to potential intruders.