A firewall using this method runs proxy applications to view common types of data (like HTTP for web-pages, FTP, SMTP or POP3 for email, etc.) before it is allowed through the firewall. While a circuit level gateway only examines theĪddress and port information contained in data it receives, not the content, anĪpplication level gateway is more in-depth. The major difference is in the way they handle information. Network Address Translation (NAT) which is a combination of circuit level gateway functions with Internet sharing.Īpplication level gateways, also known as proxies, are outwardly similar in operation to circuit level gateways in that they act as the only entrance into or out of a protected network and deny all non-requested data from outside. As you can probably guess,Ĭable/DSL home routers use this method primarily. Part of the function of this type of firewall. Gateway method will also include some method of internet sharing, since this is Software or hardware firewalls that use the circuit level The main disadvantage is that unless it is combined with some other form of filtering, any type of data requested from inside the firewall will be allowed though. All ports are closed until the firewall opens them.

When it reaches the firewall, it is compared to computer A's request to see if the IP address and the port match up, then the data is either allowed or dropped.Ī major advantage to using this method is that non-requested data from outside the firewall is not allowed in, period. Computer B receives the request, which as far as it is concerned came from the address of the firewall, and starts sending the web-page data back across the Internet.

Computer A sends the request for the web page to computer B, which is intercepted and recorded by the firewall before being passed on.

As an example of how circuit level gateways work, say computer A is in a network protected by a circuit level gateway firewall, and wants to view a web page on computer B which is outside the firewall.

Circuit level gateway