Have US ISPs ever been asked/told to block IPs (or even DNS) before? I don't think it's something the FCC could do, especially since they've abdicated most of their authority over the internet to the FTC. Even still, I doubt they have the authority to do it or the means to impose penalties for noncompliance.

Even if they could - what is a TikTok IP? I'm confident their US endpoints are AWS or Google Cloud - good luck blocking those.

I’m pretty sure that at TikTok level you need operate with a VPC and have your own range of IPs.

Also ISPs and Mobile Carriers have their own blocking and network redirection methods, like url filtering with transparent proxies, that are used to re-lane traffic based on bandwidth usage. a.k.a putting Netflix on the slow lane

I don't think the FCC will move that fast to do something this drastic but I could be wrong. This whole move will likely anger a lot of people and feel authoritarian because they aren't use to the government censoring the media they consume.

FCC is completely aligned with this administration policies. I think they would totally act fast if asked.