When Donald trump first stated he wanted to "shut down the Internet", a few people asked me if he could do that.
How can I simply explain that the Internet is a Web and not a two way stream?
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There are both technical and legal/political barriers to "shutting down the Internet."
The most important concept is that no one owns or controls the Internet. It is made up of many providers, some global, some local, all interconnected. The impetus that led to the modern Internet was to design a network that would survive a nuclear attack. If a part was destroyed, traffic would automatically be rerouted around it. So there's no one "choke point" that could be shut down. All of the major carriers (and most of the smaller ones) would have to be stopped in order to stop the Internet.
All the providers of the Internet (at least in the US) are private companies, which limits the government's ability to control them. It's difficult to imagine how companies like ATT, Verizon, Google, and Microsoft could be told to stop operating.
Many of these companies also provide telephone service as well as private networking. Those services and the Internet use the same links and equipment, so shutting down one would shut down the other. The economic consequences would be tremendous and there would be overwhelming political pressure to keep operating. Short of a complete overhaul of the American political system, I can't imagine that happening.
I think Trump's comment would be technically, legally, and politically impossible to carry out. What that may say about his knowledge and abilities as potential president I'll leave to other forums.
I would use an analogy to electrical power distribution. It can be shutdown locally for a while, but shutting power distribution down globally is infeasible (other than a major Earth catastrophe).
This analogy also illustrates why some governments can, in effect, shutdown or severely limit access to, on a country wide basis.