At 10:30 a.m. on Thursday, just about everyone in Syria discovered that they no longer had access to the Internet. As my colleague Olga Khazan explains, the most likely explanation is that the Syrian government intentionally caused the blackout to make life difficult for rebels inside the country.

So just how easy is it to sever a country's Internet connection? Over at the Web monitoring firm Renesys, James Cowie has a long post arguing that it varies from country to country. The two key variables, he writes, are infrastructure and how many companies offer connections into and out of the country:

There are 61 nations, including Syria and Libya and even Greenland, where there are only one or two providers connecting people to the outside world. "Under those circumstances," Cowie writes, "it's almost trivial for a government to issue an order that would take down the Internet. Make a few phone calls, or turn off power in a couple of central facilities, and you've (legally) disconnected the domestic Internet from the global Internet."

But countries with a more diversified Internet infrastructure are harder to cut off. It took Hosni Mubarak's government a few days to shut down Egypt's Internet. And a blackout is even harder to pull off in countries like Mexico and India. (It's interesting to see that China is ranked as a "low risk" countries. As James Fallows reported a few years ago, the Chinese government was able to set up its "Great Firewall" to censor the Internet in part because most of the country's access runs through fiber-optic cables at just three points.)

Meanwhile, it would be nearly impossible for a government to shut down the entire Internet in the United States or Western Europe. "There are just too many paths into and out of the country," Cowie writes, "too many independent providers who would have to be coerced or damaged, to make a rapid countrywide shutdown plausible to execute."

That's good to know, at least.

Further reading:

--For more on Syria, our colleagues over at WorldViews are doing excellent work. See this post by Olga Khazan or this post by Max Fisher on whether the United States can help Syria get around the blackout.

--Here's an interesting report from March on how activists in many countries are trying to decentralize their local Internet infrastructure so that it can't be shut down.