Agree. After failing the .co.jp sample test case I started wondering what is actually the 'domain' of a url, if not the first subdomain under the TLD? I figured there was no way I could find it from a url -- if I had used my brains a bit more I could have come up with .co.uk and maybe .co.au, but not .co.jp, and I have know idea how many of those suffixes I am unaware of...
Since I am not a pro by a long shot, I forfeited and checked the highest ranked answers, and I see that they fail for most other cases that I had covered (ftp://, subdomains, www.org).
I don't want to say Bad kata, because it did help me learn a lot, but it can certainly be improved.

Exactly! I was only thinking about subdomain when answerin this kata & even that is enough to disqualify the "best practice" given :(