The issue is an ancient, insecure and essential part of the internet called Border Gateway Protocol. It's a method service providers use to exchange and distribute routing information -- the instructions that point you to the web server you're looking for. The only problem is that BGP kind of works on the honor system: there's no standard in place to stop someone from putting forth a false routing path and taking a site down. It happened in 2008, when Pakistan accidental blocked YouTube. Turkey once filtered the majority of traffic from Amazon, Microsoft, CNN and other sites through its own servers. When Iran used BGP to spoof traffic away from a few hundred porn sites, the false routes spread -- blocking users as far away as Hong Kong.

It's not clear if it's an honest mistake, or if the wider blocking was intentional, but it also wasn't a careful solution. Iran's networks are a major routing center for the region, and it's easy for malicious or false BGP data to spread from Iran's sources. The issue is mostly resolved down, with exterior networks blocking the false routes -- but the event serves as a good reminder: the internet is fragile, and open to attack. Browse carefully, my friends.