A firewall is hardware or software that blocks hackers and viruses from reaching a single computer or a network of devices via the Internet. Firewalls act as a network security filtering system between the network and outside connections, monitoring and controlling incoming and outgoing network traffic based on predetermined security rules. Firewall security can be used to block unauthorized and unwanted traffic from entering the computer system or network while allowing trusted data through. Most firewalls use their own rules to filter traffic; some firewalls can develop their own filtering rules over time based on user data.
The National Institute of Standards and Technology (NIST) 800-10 divides firewalls into three basic types: packet filtering firewalls, stateful inspection firewalls, and application-level gateways (proxys). Two additional types of firewalls are circuit-level gateways, which monitor TCP handshakes, and multilayer inspection gateways, which combine packet filtering with circuit monitoring.
To learn more about Web Application Firewalls (WAFs), read our solution brief WhiteHat Sentinel Dynamic and Web Application Firewalls.