Excellent article: "How to Shut Down Internets."
First, he describes what just happened in Syria. Then:
Egypt turned off the internet by using the Border Gateway Protocol trick, and also by switching off DNS. This has a similar effect to throwing bleach over a map. The location of every street and house in the country is blotted out. All the Egyptian ISPs were, and probably still are, government licensees. It took nothing but a short series of phone calls to effect the shutdown.There are two reasons why these shutdowns happen in this manner. The first is that these governments wish to black out activities like, say, indiscriminate slaughter. That much is obvious. The second is sometimes not so obvious. These governments intend to turn the internet back on. Deep down, they believe they will be in their seats the next month and have the power to turn it back on. They believe they will win. It is the arrogance of power: they take their future for granted, and need only hide from the world the corpses it will be built on.
Cory Doctorow asks: "Why would a basket-case dictator even allow his citizenry to access the Internet in the first place?" and "Why not shut down the Internet the instant trouble breaks out?" The reason is that the Internet is a valuable tool for social control. Dictators can use the Internet for surveillance and propaganda as well as censorship, and they only resort to extreme censorship when the value of that outweighs the value of doing all three in some sort of totalitarian balance.
Related: an article on the countries most vulnerable to an Internet shutdown, based on their connectivity architecture.