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?
In the case 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.
From wikipedia -
And yes, you can create sub-sub domains, or multiple level subdomains. Check out this question from ServerFault