Or to put it another way what would need to happen in order that the internet would stop working?
Blowing up most of the root DNS servers would do it. It wouldn’t stop all Internet traffic, or even affect the physical infrastructure at all, but it would make it largely useless for a lot of people once their local caches expire.
It’d be very difficult considering the internet is made of thousands of computers and data lines and isn’t like one centralized place. That said, I think if some of the major internet backbones went down the internet would be unavailable to a large amount of people. I don’t foresee any situations where the entire internet would be shutdown for everyone, however.
The Internet isn’t a single thing. As long as phone lines exist, some parts of the Internet would continue to work.
Kill Al Gore, kill teh interwebz!
This is true, but as friedo mentioned you’ve got only a handful of root DNS servers. To a computer, a web address (like boards.straightdope.com) is meaningless. The computer has to translate the words into numbers, and it does that through the domain name system (DNS).
The way that a web address gets translated into numbers is that your computer first looks in its local “hosts” file. If it can’t find it there, it sends out a request to the DNS server your computer is configured to use (type “ipconfig/all” at a command prompt and you’ll see the address of your DNS server). If your DNS server (typically, one of your internet provider’s computers) doesn’t have a listing for that name, it sends a request further up the chain of DNS servers. At the top of the chain are the “root DNS servers”, and there are only a few of these. If you can take out all of the root DNS servers, you’ll screw up anything that relies on name to number translation, which these days is pretty much most of the internet.
Blowing up the root servers probably isn’t too bad, since much of the DNS data will be cached in the lower ranking DNS servers. The real danger is corrupting the DNS servers so that they send out bogus addresses for everything. This will make most of the internet grind to a halt in a very short time.
There are a few major links that you can sever that will cause a lot of problems. For example, if I recall correctly there’s only one major link between Australia and the rest of the world, so if you sever that link you can basically take the entire continent of Australia off of the internet. Generally speaking, though, the whole point behind the internet was to create a fault tolerant network. It is designed to handle major parts of it breaking. Your packets may end up taking a detour through outer Mongolia to get around some broken links, and things may get slow if a major link breaks, but generally speaking it is almost impossible to shut down most of the internet.
Back in the day, I used to joke that a redneck with a backhoe could do some serious damage. That was when Mae East and Mae West were the major internet exchange points. Now? Physically damaging would be pert near impossible, however, as stated above, corrupting the DNS info would have huge ramifications.
Also back in the day, computer users were more educated in the different aspects of the internet other than just the WWW, such as Newsgroups, IRC, FTP, POP-Mail, etc. That knowledge made it more apparent that the words really don’t matter, it is the numbers behind the words, in other words, IP addresses. Oh the days I spent in BIND. We are so web dependent now, we’ve almost completely lost the ability to function without it. I actually used to know the IP addresses of things, not so much anymore. This is, in my humble opinion, why a major DNS fubar would be more crippling than necessary. Imagine everyones cell phones going dead. Who the heck knows anyone’s phone numbers any more? Seen a phone book lately? How about a pay phone?
The phrase “grinding to a halt” visually captures what happens as that bad information propagates.
ETA: I’d also like a moment of silence for when geeks ruled the 'net and named things like Mae East and Mae West.
Nah, Australia would still be just fine; it’d be the rest of the world that got cut off from the Internet. 
Plus, with only one connection to the outside world, every year they’d get two extra armies for free.
Just wondering, what would happen if someone put up some dummy servers that duplicate the IP addresses of the major DNS servers? Could it even be done?
It could be done, but it would take some work. You’d have to convince everybody else’s routers that your dummy server is the “real” version of that server.
That isn’t easy if you’re just a guy on a network, because your upstream routers would not route traffic destined for an IP address which isn’t on its subnet. So to do that, you’d have to be or conspire with a major level provider. Then you’d have to do some BGP hijacking to advertise new routes for your dummy server, and hope that lots of major backbone routers update their routing tables without getting suspicious.
Today the problem is more that there are so many servers we’ve run out of good names. The Bell Labs Whippany portals were named Bonnie and Clyde. With the hated domain addressing, who cares anymore.
Voyager
{ihnp4|decvax|ucbvax}!erc3ba!sd
Only if they hold New Zealand too.
EMPs?
In spite of the distribution, all it takes is a slight modification to throw a lot of people into a tizzy. Think of recent google problems.
Do people still go into tizzies?
That happened a few years ago – all of Minnesota and parts of the Dakotas were cut off from the net.
The backbone services for all that area came through one supplier. They had carefully installed dual phone cables into their building, exiting from different sides and running down different streets and to different central offices.
But as they discovered, at one point a mile or so away, both phone cables ran under the same bridge over the Mississippi river, right next to each other. (About 2 bridges downstream from the I-35 bridge that collapsed recently.)
And during a Minnesota winter, some homeless person living under that bridge started a campfire to try to keep warm. But the fire got out of control, and burned up the cables running underneath that bridge – including both of the redundant internet connections. [That routing was quickly corrected – now they have made sure that the connections are physically separated all along the route – they actually go to 2 different cities, in 2 separate area codes now.]
So much of Minnesota, the Dakotas & parts of Wisconsin were cut off from the net. It was confusing; you could reach in-state websites, if you had previously accessed them so your machine had the DNS address, or if a local subsidiary DNS server could translate the address for your machine. But if it was a website hosted somewhere else, you couldn’t reach it.
But it’s really hard to know where a site is hosted. Many strictly local companies or organizations actually had their website hosted far away, so you couldn’t reach them. No real pattern that people could see as to what parts of the internet were reachable. And email, too: emails to someone on your own ISP or a nearby local ISP would get through, but any Aol or Yahoo email addresses weren’t reachable at that time. The internet was used by a much smaller group at that time, and much more computer-literate group. If that happened today, there would probably be much more confusion – and much more outcry!
That’s happened three times so far in the last 30 years here, where some farmer with a backhoe dug up the single fiber cable that all communications with our county use to connect with the outside world.
Even celphones, which have to connect to landlines at the towers, and the landlines go thru the same fiber. So all phones and data communications were cut off at once. I was able to climb on my roof and barely connect my celphone to a tower on the other side of the break, about 20 miles away. All the other towers automatically shut down when their landline connection went dead.