Egypt Unrest: How do you shut down Internet service in an entire country?

"To silence dissidents, the Egyptian government made a move Jan. 27 that has no precedent: It turned off the Internet nationwide. How did they do it — and could the same thing happen here? According to David Clark, an MIT computer scientist whose research focuses on Internet architecture and development, a government's ability to control the Internet depends on its control of Internet Service Providers (ISPs), the private sector companies that grant Internet access to customers."