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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,, 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.

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up vote 3 down vote accepted

Look at the RFC2181, “Clarifications to the DNS Specification”, section 11, “Name syntax”:

The DNS itself places only one restriction on the particular labels that can be used to identify resource records. That one restriction relates to the length of the label and the full name. The length of any one label is limited to between 1 and 63 octets. A full domain name is limited to 255 octets (including the separators). The zero length full name is defined as representing the root of the DNS tree, and is typically written and displayed as ".". Those restrictions aside, any binary string whatever can be used as the label of any resource record.

Implementations of the DNS protocols must not place any restrictions on the labels that can be used. In particular, DNS servers must not refuse to serve a zone because it contains labels that might not be acceptable to some DNS client programs.

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Thank you, after scanning through the even later updates I think this is exactly what is was looking for. Seems like my co-worker is merely 16 years behind after all.. thanks! – Karma Fusebox Feb 15 '13 at 15:00

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