"Under what enumerated power of the federal government do you see this regulation being enacted?"
The Darpanet is Federally owned and funded. It would just take a regulation, albeit a very carefully crafted regulation, to prohibit porn from being transmitted over Federally owned servers, routers, and switches. The government simply has to excuse itself from the porn industry. Technically, it would not be a speech limitation of any kind, but a limitation of network service.
Certifiably private networks would be unaffected.
Certifiably private networks would be unaffected.
The government can do that now. There are segments of the IP address space set aside for government use, and traffic on those segments flows through government controlled routers. They can filter any IP address they want, and there are probably regulations in place that state that government owned servers shouldn't be used as porn servers. The vast majority of the internet servers, routers, switches, and even backbone segments are privately owned.