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I was wondering, are co.uk domains and others with an additional dot subdomains? If not, why not, and if so, can you create sub-sub domains?

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up vote 8 down vote accepted

uk is a TLD (top level domain). uk is the ccTLD (country code TLD) for the United Kingdom. All uk domains are controlled by Nominet. Nominet imposes additional restrictions on the registration of these domains. No-one can register a subdomain directly under the TLD uk. They must register a domain under one of the defined subdomains .co.uk, .org.uk and one or two others.

Yes, co is a subdomain of uk in the case of .co.uk. co is a SLD (second level domain).

In the case of example.co.uk then example (a third level domain) is a subdomain of co (a second level domain or SLD) which is a subdomain of uk (a top level domain or TLD).

foo.bar.example.co.uk - foo (fifth level domain) is a subdomain of bar, which is a subdomain of...

You can have many many subdomains (upto 127 I seem to recall), however, there are additional limits as to the length of the full domain which you would probably exceed first.

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.co.uk and the likes are country code second level domains (ccSLD).

From wikipedia -

A country code second-level domain (ccSLD) is a second-level domain to a country code top-level domain. A ccSLD may be reserved by the domain name registry for the registration of third-level domains or assigned to a third party as a subdomain. Many country code domain registries implement domain name classes at the second level underneath their ccTLD, such as are present in the original generic top-level domains com, net, and org, which were intended for commercial entities, network operators, and non-profit organizations, respectively.

And yes, you can create sub-sub domains, or multiple level subdomains. Check out this question from ServerFault

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