I read an article in Dawn’s Sci-tech World in which global outage was described very nicely: “Not content at banning YouTube in just Pakistan however, one service provider, namely the PTCL, decided that the best way to filter the website was to re-rout the IP address block that YouTube’s servers are into a ‘black hole’. This routing information escaped from the PTCL to its main ISP PCCW in Hong Kong, which propagated the route to the rest of the world. So any packets for YouTube from around the world ended up in the PTCL’s black hole instead, resulting in a global outage of YouTube.”