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A firewall protects your computer from incoming connections. Without a firewall there are many ways in which someone could connect to a PC without the user of the PC being aware, and this might allow a hacker access to files and even the ability to run software on the PC. Software firewalls (i.e. those listed here) will generally also offer control over outgoing connections, this means that they will notify you of which applications are trying to make a connection to the internet (or local network) and allow you control over which should be allowed to do so. Serious thought should also be given to installing anti-virus software, which is equally if not more important.

The following software firewall products are available free for for personal use. (A search of the web will reveal other commercial firewall solutions.) There may be some conditions attached to their use and the definition of "personal use" may vary between products, so check their terms and conditions:

ZoneAlarm - Download Free Firewall Software
ZoneAlarm is a firewall program that protects your computer from hackers while you are online. Free for personal use, with free software updates. Windows Vista and Windows XP. Very effective and easy to use. ZoneAlarm also provide a suite of protection tools, including anti-virus.

ZoneAlarm has the concept of trusted and non-trusted networks. This is very useful for anyone with their own local network (which a growing number of home users now have), because it allows different rules to be created for an application according to whether it is trying to access the trusted or non-trusted network. For example an application could be configured to allow access over a local area network but not to the internet.

The Professional version (which is not free) supports additional features such as support for connection sharing.

Filseclab Personal Firewall
Filseclab Personal Firewall is a free firewall.

One thing which which sets this firewall apart from the others is that you can buy the software source code, so in theory you could then compile and customise it yourself. This might be of serious interest to some (particularly those with a technical background), but probably would not be of interest to most users.

The Comodo firewall is a very professional looking and free firewall. Supports Windows 7, Windows Vista and Windows XP.

No apparent limitations on use. Most free firewalls are limited to one per household, non-commercial use, but neither of these restrictions apply to the Comodo firewall.

The Comodo firewall also comes with an interesting idea, which is to "sandbox" applications until they have been approved. This means that those applications which you have yet to approve are run in a limited environment where they are less likely to do any harm should they prove to be harmful. This does also mean that some applications are unable to run or run with limitations until they have been approved.

Windows XP, Windows Vista and Windows 7 come with an in-built firewall. Whilst this should prevent against many external threats to your PC it does not:

A firewall on a server is just as important as a firewall on a personal computer. A server firewall will protect the server from connections to all but permitted ports and applications. There are many firewall products available for servers, but since these are typically paid applications and not free ones they are outside the scope of this article. (Windows servers all come with free firewalls.)

It should be appreciated that an incorrectly configured firewall will also block inbound connections which you might want to allow. This is probably more likely to be an issue on a server where a poorly configured firewall could for example block inbound HTTP (web) requests, which would clearly be a bad thing for a web-server. Fortunately these issues are normally quick to resolve once the firewall is identified as needing reconfiguring.