To reduce security risks, appropriate network access policies should be defined as part of business strategy. Firewalls can be used to enforce such policies. A firewall is a network interconnection element that polices traffic flows between internal or protected networks and external or public networks such as the Internet. Firewalls can also be used to segment internal networks.

The application of firewall technologies only represents a portion of an overall security strategy. Firewall solutions do not guarantee 100% security by themselves. These technologies must be complemented with other security measures, such as user authentication and encryption, to achieve a complete solution.

The three technologies that are most commonly used in firewall products are packet filtering, proxy servers, and hybrid. These technologies operate at different levels of detail and provide varying degrees of network access protection. Therefore, these technologies are not mutually exclusive. A firewall product might implement several such technologies simultaneously.