[yyyk]

>If you had a block of 20 IPv4 addresses, I don't see anything stopping you from doing the exact same thing.

A block isn't necessary at all, even on IPv4. Traceroute can easily show internal address or just be lied to. Just recently we've seen a different example of this on HN:

https://news.ycombinator.com/item?id=32566730

[stonekyx]

IP addresses can be lied about, but reverse DNS still needs your ownership of those IP addresses if I understand it correctly. One would need the provider of those IP addresses to setup PTR records for them.

[yyyk]

You're right. I didn't consider we don't control the other side's DNS server.