An example of a type of service which a firewall would use circuit level filters for is an outgoing Telnet connection Even though this type of filtering is not as secure as Application Level Filtering, it has less overhead and is therefore faster. All three filters have their own advantages and disadvantages. Therefore, good firewalls implement all three kinds of filters to some degree in their design. This allows the engineers to balance the benefits of a given filter against its drawbacks. Along with filtering levels, firewalls are set up in a variety of physical configurations.
Most firewalls are dual-homed, meaning that they have two physical network devices.
The firewall sits between two physical networks, and controls what information can be passed between those networks. All data must pass through the firewall, since there are no other available paths between the networks for data to pass. The implications of this are important, since by doing this, these services can be closely monitored by the firewall, while not requiring connections to pass through the firewall. Circuit Level Gateways - Firewalls also implements many circuit level gateways to allow for transparent connections from hosts inside the firewall to sites outside. The firewall allows strict control over which services may pass through it, even including control over the times of day when these connections can be made.