How does the firewall monitor the traffic? In principle, the firewall works like a doorman for data packets: Each packet is inspected to see if it is allowed to pass the door of the network (the firewall) in the desired direction or not. This firewall inspection makes use of various criteria referred to as "rules" or "guidelines". Different types of firewalls are distinguished according to the type of information that is used to create the rules and that is inspected by firewall operations.
Above all, the aspect of central positioning is important: Only when all of the data traffic between the inside and outside passes through the firewall can it reliably fulfill its task. Any alternative path would reduce or even disable the security of the firewall. This central location of the firewall also makes maintenance easier: A firewall as a common transition between two networks is easier to maintain than a "personal firewall" on each computer in the LAN.
