A Firewall acts as a barrier between a trusted system or network and outside connections, such as the Internet. However, a computer firewall is more of a filter than a wall, allowing trusted data to flow through it. Both Windows and OS X (for the Mac) include built-in firewalls, but more advanced firewall utilities can be installed with Internet security software. Most firewalls use a combination of rules to filter traffic, such as blocking known threats while allowing incoming traffic from trusted sources. A firewall can also restrict outgoing traffic to prevent spam or hacking attempts.