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I have seen domains such as 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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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

No. At the very least after the second level domain ( 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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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

Simply put (in addition to restrictions mentioned in other answer):
|      |     |
|      |     .mobi - TLD (Top-level domain) => Currently ~253 to pick from
|      |
| - Domain name => Can be anything that hasn't been registered
| - 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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