Firewalls provide security to network systems by controlling the flow of incoming and outgoing traffic and preventing unauthorized access. The guidelines document of the National Institute of Standards and Technology (NIST) defines firewalls as "devices or programs that control the flow of network traffic between networks or hosts that employ differing security postures" (Scarfone & Hoffman, 2009).

Firewalls are deployed extensively by businesses, educational institutions, government organizations, and end users to prevent cyberattacks and to protect sensitive information.

There are two types of firewall implementation: software and hardware. Software firewalls are installed on individual systems or machines, whereas hardware firewalls are implemented using specialized hardware equipment (on network switches or routers) to provide security to all connected machines. According to one expert:

A firewall can exist as hardware or software (or both). A hardware firewall is a device that is connected to the network and filters the packets based on a set of rules. A software firewall runs on the operating system and intercepts packets as they arrive to a computer (Bourgeois, 2014).

References

Bourgeois, D. T. (2014). Information systems for business and beyond. The Saylor Academy. http://www.saylor.org/site/textbooks/Information%20Systems%20for%20Business%20and%20Beyond.pdf

Scarfone, K., & Hoffman, P. (2009). U.S. guidelines on firewalls and firewall policy: Recommendations of the National Institute of Standards and Technology: Special Publication 800-41.. National Institute of Standards and Technology. http://csrc.nist.gov/publications/nistpubs/800-41-Rev1/sp800-41-rev1.pdf