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I've googled this a few different ways but I haven't been able to find anything official.
What is the minimum subdomain length?
From what I understand, a domain has to be at least 3 characters, but I haven't been able to find anything about the minimum for a subdomain.
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RFC 1034 section 3.1 specifies that labels (parts of DNS names) can be from 0 to 63 octets long, and the zero-length one is reserved for the root zone only. So in practice, the limits are 1 to 63 characters.
A subdomain can be up to 255 characters long, but if you have multiple levels in your subdomain, each level can only be 63 characters long.
A domain name consists of minimum two and maximum 63 characters. All letters from a to z, all numbers from 0 to 9 and a hyphen (-) are possible. A domain name mustn't consist of a hyphen (-) on the third and fourth position at the same time.
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.
RFC 1034 section 3.1 specifies that labels (parts of DNS names) can be from 0 to 63 octets long, and the zero-length one is reserved for the root zone only. So in practice, the limits are 1 to 63 characters.
178
Considering the fact that I've seen plenty of sites with just "m" as a subdomain (for mobile versions of pages), I would guess that the minimum length is 1.
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