Windows

Windows XP provides the Internet Connection Firewall, and beginning with Service Pack 2, Microsoft has encouraged enabling the firewall.

Other popular firewall/security suites include Zonelabs ZoneAlarm, Norton Internet Security or Personal Firewall, McAfee Internet Securtity Suite or Personal Firewall.

If you have any of these programs or any other firewall program running on your computer, you should answer Yes.

Mac OS X

The most common Mac OS X firewall is the built-in firewall which can be enabled from the Sharing System Preferences. If you have enabled this firewall or installed a separate firewall, you should answer Yes.

Linux

Linux users have many choices for firewalls, such as ipchains or iptables. If you are using either of these popular rule-based firewalls, or any other firewall program, you should answer Yes.