A few ways around a "private network shutdown", which would undoubtedly mean controlling US based DNS servers to block IP addresses assigned to commercial vendors, would be to use other DNS servers, to have a gov't or major university job, to have a dialup connection to another country's ISP...and many others.

The biggest issue with government invading this field is that the internet is a utility and a communications venue. Shutting down any portion of the IP ranges would wreak havoc on businesses and individuals using VoIP phone systems and e-commerce. Comparing it to 9/11 grounding of aircraft is ridiculous, it's not at all the same. Grounding aircraft inconvenienced several thousand people, it didn't shut down workplaces for millions.