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I noticed that looking at registrars some top-level-domains had different rules it seems. Like on some domains I can get a 3 letter second-level-domain but others say minimum 4 letters. Is there supposed to be some standard rule or is it just each top level registrar makes their own rules?

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Each makes their own rules. There is no standard.

Many of the older ones, for example country-level specific, may want to restrict shorter ones to avoid confusion. For example, doesn't allow two-character domain names.

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It depends on the registrar. For example, Canada used to require hierarchical domain names:

If your company was in only one province, you had to get use a provincial sub-domain, e.g. But if you were a national organization, you could get just

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