I’m sure this has been on the minds of many following the ongoing events in Egypt. An io9 article discusses the possibility:
With the threat of today’s protests looming in Egypt, on Thursday Egyptian authorities cut the nation off the internet. No online communication could pass in or out of the country. We investigated whether a similar lockdown could happen in America.
How the Egyptian government erased its citizens from the internet
No one is completely certain what happened to the Egyptian internet, but it appears that the shutdown started off early in the week with the country blocking Twitter and Facebook access for those within its borders. Then, shortly after Thursday midnight local time, the country simply disappeared from the internet. With a few exceptions like the stock exchange, Egyptian websites and services were unreachable; the network traffic over Egyptian borders dropped by an astonishing 90 percent. Cell phone networks were also down. Today Vodafone confirmed that all mobile providers been “obliged” by the Egyptian government to shut off service to customers in that country.
How did it happen? Most likely, according to experts, the Egyptian authorities made a few phone calls to the country’s handful of large internet service providers. Their request would have been simple: Make it impossible for Egyptians to communicate with other countries and each other using the internet. But carrying out this order isn’t like hitting an off switch.
To erase Egypt, those providers would have to corrupt routers, nodes in the internet which direct data traffic. Each router helps traffic along by advertising the many IP addresses it knows using a system called border gateway protocol (BGP). When you visit a website in Egypt, your internet provider uses BGP to ask an Egyptian router, “Hey, how do I get to this Egyptian blog?” The router responds by using BGP to send you on your merry way to the right address. BGP is basically the border language that helps different parts of the internet speak to each other.
But early Friday morning, thousands of routers in Egypt had their minds wiped. Suddenly they had no idea where anything was on the Egyptian internet. When your internet provider looked for web addresses inside Egypt, it found nothing. Egyptian routers no longer gave meaningful answers to BGP requests for border crossings.
Article continues at io9.