IP multicast routing and internetworking is a thing. There used to be working one to many internet TV based on multicast in the 90s (was called Mbone). There's contemporary experimental use of routed multicast on public internets too (see eg https://meetings.internet2.edu/2019-technology-exchange/deta...)
edit: seems the IETF are currently in process of deprecating any-source multicast and recommending people focus on keeping single-source multicast working well in as many networks as possible: https://datatracker.ietf.org/doc/rfc8815/
I was on the local MBone segment at the NASA base (though a Navy installation) I worked at in the late 90s. It pretty much worked like you would expect, some interesting things that happened:
- Multicast frames caused our FDDI switches to reboot, had to filter everywhere those were used
- When setting up our local CoffeeCam, we used Multicast video stream -- this would have been accessable from anywhere (which would cause a security incident, no one can know when the Rear Admiral drinks coffee) so we were careful to monitor outbound advertisements and used a low TTL
- SDR, VIC, VAT, RAT, and all those other tools for doing multicast stuff worked but were very clunky
- NASA TV was broadcast over the MBone and times they would go to break in other broadcasts were continued to stream, I think various cameras were available
edit: seems the IETF are currently in process of deprecating any-source multicast and recommending people focus on keeping single-source multicast working well in as many networks as possible: https://datatracker.ietf.org/doc/rfc8815/