In 1993, Internet pioneer John Gilmore said "the net interprets censorship as damage and routes around it", and we believed him. In 1996, cyberlibertarian John Perry Barlow issued his 'Declaration of the Independence of Cyberspace' at the World Economic Forum at Davos, Switzerland, and online. He told governments: "You have no moral right to rule us, nor do you possess any methods of enforcement that we have true reason to fear."

At the time, many shared Barlow's sentiments. The Internet empowered people. It gave them access to information and couldn't be stopped, blocked or filtered. Give someone access to the Internet, and they have access to everything. Governments that relied on censorship to control their citizens were doomed.

Today, things are very different. Internet censorship is flourishing. Organizations selectively block employees' access to the Internet. At least 26 countries -- mainly in the Middle East, North Africa, Asia, the Pacific and the former Soviet Union -- selectively block their citizens' Internet access. Even more countries legislate to control what can and cannot be said, downloaded or linked to. "You have no sovereignty where we gather," said Barlow. Oh yes we do, the governments of the world have replied.

Access Denied is a survey of the practice of Internet filtering, and a sourcebook of details about the countries that engage in the practice. It is written by researchers of the OpenNet Initiative (ONI), an organization that is dedicated to documenting global Internet filtering around the world.

The first half of the book comprises essays written by ONI researchers on the politics, practice, technology, legality and social effects of Internet filtering. There are three basic rationales for Internet censorship: politics and power; social norms, morals and religion; and security concerns.

Some countries, such as India, filter only a few sites; others, such as Iran, extensively filter the Internet. Saudi Arabia tries to block all pornography (social norms and morals). Syria blocks everything from the Israeli domain ".il" (politics and power). Some countries filter only at certain times. During the 2006 elections in Belarus, for example, the website of the main opposition candidate disappeared from the Internet.

The effectiveness of Internet filtering is mixed; it depends on the tools used and the granularity of filtering. It is much easier to block particular URLs or entire domains than it is to block information on a particular topic. Some countries block specific sites or URLs based on some predefined list but new URLs with similar content appear all the time. Other countries -- notably China -- try to filter on the basis of keywords in the actual web pages. A halfway measure is to filter on the basis of URL keywords: names of dissidents or political parties, or sexual words.

Much of the technology has other applications. Software for filtering is a legitimate product category, purchased by schools to limit access by children to objectionable material and by corporations trying to prevent their employees from being distracted at work. One chapter discusses the ethical implications of companies selling products, services and technologies that enable Internet censorship.

Some censorship is legal, not technical. Countries have laws against publishing certain content, registration requirements that prevent anonymous Internet use, liability laws that force Internet service providers to filter themselves, or surveillance. Egypt does not engage in technical Internet filtering; instead, its laws discourage the publishing and reading of certain content -- it has even jailed people for their online activities.

The second half of Access Denied consists of detailed descriptions of Internet use, regulations and censorship in eight regions of the world, and in each of 40 different countries. The ONI found evidence of censorship in 26 of those 40. For the other 14 countries, it summarizes the legal and regulatory framework surrounding Internet use, and tests the results that indicated no censorship. This leads to 200 pages of rather dry reading, but it is vitally important to have this information well-documented and easily accessible. The book's data are from 2006, but the authors promise frequent updates on the ONI website.

No set of Internet censorship measures is perfect. It is often easy to find the same information on uncensored URLs, and relatively easy to get around the filtering mechanisms and to view prohibited web pages if you know what you're doing. But most people don't have the computer skills to bypass controls, and in a country where doing so is punishable by jail -- or worse -- few take the risk. So even porous and ineffective attempts at censorship can become very effective socially and politically.

In 1996, Barlow said: "You are trying to ward off the virus of liberty by erecting guard posts at the frontiers of cyberspace. These may keep out the contagion for some time, but they will not work in a world that will soon be blanketed in bit-bearing media."

Brave words, but premature. Certainly, there is much more information available to many more people today than there was in 1996. But the Internet is made up of physical computers and connections that exist within national boundaries. Today's Internet still has borders and, increasingly, countries want to control what passes through them. In documenting this control, the ONI has performed an invaluable service.

This was originally published in Nature.

The first three have been blamed on ships' anchors, but there is some dispute about that. And that's two in the Mediterranean and two in the Persian Gulf.

There have been no official reports of malice to me, but it's an awfully big coincidence. The fact that Iran has lost Internet connectivity only makes this weirder.

EDITED TO ADD (2/5): The International Herald Tribune has more. And a comment below questions whether Iran being offline has anything to do with this.