Why did the web come to a grinding halt last thursday? Wired.com have a great article explaining it clearly.
"The problem began around 8 a.m. EDT. Baker said in a statement that the company had fully restored service by 5:15 p.m. Thursday evening. Preliminary investigation by UUNet indicates the problems were caused by "a route table issue." Sounds simple, but imagine an airport that"s having an air traffic controller issue, and you"ll have an idea of what happened at UUNet. Route tables direct data from one major network to another or from one area of a network to another area.
UUNet is a vast, high-speed network. About half of the world"s Internet traffic -- including about 70 percent of all e-mails sent within the United States and half of all e-mails sent in the world -- passes through UUNet. The backbone of the Internet is built from these large networks. The Internet was designed to be fault tolerant, to route information around downed or clogged networks. But when the router tables that direct the data aren"t accurate, "bedlam reigns on the network," according to Mike Sweeney, owner of the network consulting firm Packetattack.com. "