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I have seen domains such as www.uk.domain.mobi. Is there no restriction in terms of what is allowed after www?

Can I call a subdomain anything I like and is there no dot-separated word limit?

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1  
Doesn't matter what the beginning is - resolution starts from the end (.mobi) –  OrangeDog Nov 20 '12 at 16:02
    
Thanks! Didnt know it was RTL –  codecowboy Nov 21 '12 at 21:21
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2 Answers 2

No. At the very least after the second level domain (domain.mobi) you are completely free to choose additional subdomain names. Even www is such an arbitrary name. Of course, your local laws might restrict you, and there are some technical restrictions:

  • max.127 levels
  • max 253 characters for the complete name
  • 63 characters for a sub-name (like www).

See this for more infos.

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3  
In case it's not clear from Sven's otherwise-excellent answer, you're parsing the domain name from the wrong end. the .mobi end is tightly-fixed, highly-regulated by IANA; but as you go further and further towards the beginning of the string, to the www. end, it becomes more and more a matter of choice for the domain registrant. –  MadHatter Nov 20 '12 at 8:24
    
Yep, I didnt know this. 8+ years in IT. Am suitably ashamed :( –  codecowboy Nov 21 '12 at 21:23
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Simply put (in addition to restrictions mentioned in other answer):

www.uk.domain.mobi
|      |     |
|      |     .mobi - TLD (Top-level domain) => Currently ~253 to pick from
|      |
|      domain.mobi - Domain name => Can be anything that hasn't been registered
|
www.uk. - Sub-domain(s) => Can be anything you want after registering the above.

List of TLDs

Can I call a subdomain anything I like and is there no dot-separated word limit?

Yes, once in control of the DNS records you can create subdomains, sub-subdomains, sub-sub-subdomains, etc.

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So what is www then? An arbitrary subdomain name? –  codecowboy Nov 21 '12 at 21:20
    
Yes, it stands for "world wide web" and is not needed for the browser to know that http:// maps to port 80 by default. Just a relic from the past.. there are arguments both for (1,2) and against (1) keeping it. Serverfault opts for redirecting to a naked (no-www) domain name. –  deizel Dec 13 '12 at 16:22
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