After a steamy video of Brazilian model Daniela Cicarelli leaked onto the Internet, a judge ordered YouTube banned in Brazil until the site took down all copies of the video. (Click here to see the video, which may not be appropriate for the workplace.) How does a government block a Web site?
Any way it can. For starters, the government can tell its Internet service providers to cut off their customers from a certain site. Through a process called "packet filtering," ISPs can enforce a government blacklist by scanning data entering the ISP's network from abroad and then blocking information from certain domains or IP addresses. (Blacklists are usually top secret. Usually.) This method tends to be inexact, because many IP addresses host thousands of sites, which may get inadvertently banned. When Pakistan tried to embargo a handful of blogs hosted by Blogger.com during the Muhammad cartoon uproar, every single blog on the site was blocked. Some ISPs also filter less efficiently than others, so a ban might not be enforced the same way in every part of a country. (In Brazil, for example, people in Rio de Janeiro could see the beach sex video while people in Brasília were cut off.)
Filtering becomes a lot simpler when a country's networks are centralized, in which case all ISPs receive information through the same hub, sometimes called a "gateway." Blocking information at this level makes filtration uniform across the country, instead of leaving it up to each ISP. Pakistan, Saudi Arabia, and Tunisia all have centralized networks; the United States does not.
Some countries opt for more sophisticated techniques that look for specific words in a site's Web address. With software like SmartFilter, a government can censor the results of a search engine query, since sites like Google and Yahoo! append the search terms to their URLs. This also helps keep out mirror sites, which usually incorporate the original sites' URLs in their own. Some filtering programs will allow a government to ban entire categories of sites, like pornography, gambling, or advocacy groups. This often means a country doesn't even know what sites it's blocking. In one case, Saudi Arabia unblocked two gay and lesbian Web sites that had been mistakenly categorized under pornography.
Filtration is becoming even more sophisticated—some programs have even tried to filter out pornographic images based on perceived levels of flesh color. But so is anti-filtration. Determined Web surfers can try to access a computer in another country as a proxy for getting to sites they want. The filter knows they're visiting the benign server site, but it doesn't know they're using it to access banned information. Governments sometimes discover these servers and ban them, too, resulting in an ongoing cat-and-mouse game. (Want to turn your personal computer into a bypass server? Download a program here.) Programs called "anonymizers" let you circumvent filtering systems and browse undetected. Another option is to dial up another country and use the phone line as a modem, which lets you access Web sites under the foreign country's rules. Linguistic differences provide yet another loophole: Most countries only censor sites in their local language, so CNN is accessible just about everywhere. In Iran, you can read any BBC site except BBC Persian.
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Explainer thanks Nart Villeneuve of OpenNet Initiative.