A firewall is a Network security system, either hardware or software based, that controls incoming and outgoing network traffic based on a set of rules. Acting as a barrier between a trusted network and other un-trusted networks such as the Internet or less-trusted networks such as a retail merchant’s network outside of a cardholder data environment, a firewall controls access to the resources of a network through a positive control model. This means that the only traffic allowed onto the network defined in the firewall policy is all other traffic is denied.
