The CloudFlare blog has an excellent post about the Syrian Internet shut off today:

To begin, all connectivity to Syria, not just some regions, has been cut. The exclusive provider of Internet access in Syria is the state-run Syrian Telecommunications Establishment. Their network AS number is AS29386. The following network providers typically provide connectivity from Syria to the rest of the Internet: PCCW and Turk Telekom as the primary providers with Telecom Italia, TATA for additional capacity. When the outage happened, the BGP routes to Syrian IP space were all simultaneously withdrawn from all of Syria’s upstream providers. The effect of this is that networks were unable to route traffic to Syrian IP space, effectively cutting the country off the Internet.