Software installed in a user's computer that offers protection against unwanted intrusion and attacks coming from the Internet. Personal firewalls are available from numerous security vendors, and Windows XP includes its own (see Windows Firewall).

Router Firewalls
For home and small business networks, a router with a built-in firewall is often used, and personal firewalls can be disabled, although they are often kept running as well. The central firewall provides easier administration than a firewall in each user's machine.

Enterprise Firewalls
In an enterprise, a firewall is often a stand-alone appliance. In addition, a comprehensive security program may comprise several firewall functions in different venues, each providing a different level of protection. See firewall, software firewall and IDS.