I had an argument with a co-worker who insisted that domain names consisting entirely of numbers should not be valid. In reality however, they exist and work flawlessly under any top level domain I've tried and I'm not aware of any DNS- or webserver that would state a problem with handling them.
(just look at reverse DNS, in-addr.arpa, which would totally not work if there were no number-only labels)
So far on the practice/reality. In theory I see nothing that would exclude number-only labels from DNS either. So the only point that could bring my co-worker to his point of view is some weird standard definition. And now I'm on the hunt for that one.
I stumbled upon RFC 1035, where section 2.3.1 could be interpreted the way, that numbers-only are not recommended for subdomain names, just numbers with letters in front.
Which of the many updating RFCs contain more liberal approaches on naming or were there really no updates concerning this issue?
If this one, from 1987, is latest on this topic, one can conclude that in fact any TLD-registry on the planet is disregarding a latest RFC on their main business. I would find that quite hard to believe.