A firewall is a software utility or hardware device that acts as a filter for data entering or leaving a network or computer. You could think of a firewall as a security guard that decides who enters or exits a building. A firewall works by blocking or restricting network ports. Firewalls are commonly used to help prevent unauthorized access to both company and home networks.
Software firewalls are designed to protect a computer by blocking certain programs from sending and receiving information from a local network or the Internet. By default, most programs are blocked by the firewall but can be excluded through the firewall settings.
Hardware firewalls are found on most network routers and can be configured through the router setup screen. The image at the top of this page shows the ZyXEL ZyWALL, a Unified Security Gateway with a firewall and other security features.