[TheSoftwareGuy]

It should really be more like how the internet works with IP addresses.

Its easy to send an IP packet with a source address that is whatever you want, but if you do that the receiver is going to reply to someone else and you won't be able to establish two way communication.

[musicale]

Ideally the ISP does basic ingress policing and doesn't accept packets from source addresses outside its range.

Yes, I know most ISPs don't do this. But they could if they wanted to.