internet shutdown

I don't think someone or a group can do that because there are different Internet Service Providers all over the world and they have different kinds of securtiy used in order to avoid hackers.. that's just what i think..

Total shutdown probably not. But service disruption, highly possible. Internet is simple inter-connecting a number of networks so that you can access a website created by somebody in the other side of the world. Now, there are many network path from one place to another but as the distance get further, the number of redundant links decrease. A few years ago, there was an earthquake in Asia where a number of underwater cables are damaged. During that time, the network speed is greatly reduced when we had to access websites hosted in Europe/US from Asia. So, can the Internet be actually totally shutdown? Not quite likely as ISP have redundant links to handle that. But then some major links, when damaged, can seriously affect the network performance.

Yes, it's more than possible. All you need is a large enough botnet. Given enough computers participating in a DDOS attack, you can take down the major DNS servers so that the remaining connected computers don't know how to resolve each others' addresses. Or you could physically take out the facilities hosting those servers. I doubt it will become a target any time soon, at least on the large scale. Any group smart enough to take it out will be smart enough to understand just how much of their own functioning depends on it.