At the risk of sounding overly technical, I tried to understand how Pakistan could shut
down the Internet as the government objected to what it considered disparaging to the Prophet Mohammed. Pakistan used a Border Gateway Protocol (BGP) injection. In lay terms, Pakistan changed its Internet routing information for YouTube so that requests would go to Pakistani servers, not YouTube's. But the changed information was also sent to Pakistan's own Internet provider, Hong Kong's PCCW, which accepted it and passed it along to the rest of the world. And the result was that almost instantaneously, YouTube disappeared, for you and I.
The nefarious Pakistani deed took about two hours to correct.
BGP injections won't be fixed nor will they likely be. At times it is by accident, other times spammers or hijackers are sabotaging an address. More ominously, governments keep attempting to censor Web sites, and in the climate we are in, that kind of sabotage is likely to happen more and more.
The basic issue is that for the big network providers, relations between them still runs on trust. And as long as one of them sends out routing information to the others, we presume it to be true.