The free exchange of information over the Internet has proven to be a threat to the social and political control that repressive governments covet. But rather than ban the Internet (and lose valuable business opportunities), most repressive governments seek to limit their citizens' access to it instead.

To do so, they use specialized computer hardware and software to create firewalls. These firewalls prevent citizens from accessing Web pages - or transmitting emails or files - that contain information of which their government disapproves.

U.S. Internet users may be familiar with the kind of firewalls that limit unauthorized access to confidential information kept on personal, business, or government computers. But the type of country-wide firewalls this article will look at are broader, and more pernicious. In countries like Saudi Arabia, Iran, China, Myanmar, Singapore, and at least 20 others, the only way to access the Web is through a firewall.

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