There are only 13 dns root name servers in the world located in the different countries . Dns is used to resolve the domain to the ip addresses. So if terrorists makes all the Dns root name servers crash simultaneously then Dns servers will not be able to resolve the domain to the ip address. So after small amount of time Dns look ups will be failing due to the crash, then after an hour or so more numbers of Dns look ups will fail and after 24 hours most of the Dns look up will fail. You can only access the internet if you know exactly the resolved ip address of the particular website
