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.com,.us are TLD,top level domain, and co.uk are SLD,second level domain.

Are there "third level domain, fourth level domain" and so on?

How many different TLDs and SLDs are out there?

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6 Answers 6

This link seems to have the most relevant detail Wiki - DNS_Label - Domain name formulation

The hierarchy of domains descends from right to left; each label to the left specifies a subdivision, or subdomain of the domain to the right. For example: the label example specifies a subdomain of the com domain, and www is a subdomain of example.com. This tree of subdivisions may consist of 127 levels.

Each label may contain up to 63 characters. The full domain name may not exceed a total length of 253 characters. In practice, some domain registries may have shorter limits.

You asked in a comment

sub1.google.com is also owned by owner of google.com,but that's not the case for news.bbs.co.uk,say it's not owned by owner of co.uk,but bbs.co.uk!

Any owner of a domain may delegate authority to anyone else. If I was the owner of example.com, I could delegate authority for sub.example.com to you. Anyone can delegate their subdomains to other organizations. If you curious about the policies for a particular domain you'll probably need to find the correct contact for that domain.

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http://en.wikipedia.org/wiki/Domain_name#Second-level_and_lower_level_domains

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But sub1.google.com is essentially google.com.While it's not the case for news.bbs.co.uk,its domain is bbs.co.uk –  PHP Feb 4 '10 at 2:16
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sub1.google.com is not essentially google.com, if an A record for abc.com and an A record for sub1.abc.com that are the same then there won't be any difference in the website you visit unless the web server implements host header filtering. It depends where people point their domains to. –  ta.speot.is Feb 4 '10 at 3:32
    
I mean sub1.google.com is also owned by owner of google.com,but that's not the case for news.bbs.co.uk,say it's not owned by owner of co.uk,but bbs.co.uk! –  PHP Feb 4 '10 at 4:25

I'm not aware of a technical limit to the number of levels but to be sure you should consult the RFCs relating to DNS.

An example such as news.bbs.co.uk, as you used in a comment, shows four levels already.

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I just jumped onto my DNS and created this:

alt text

That's 29 levels and counting, so I guess as John said, you can just keep going and going and going

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"the full domain name may not exceed a total length of 253 characters" apparently, so assuming a very short top+second (for example is.gd) you could have 126 levels (including the .is and .gd) if each sub division is only a single character. –  David Spillett Feb 6 '10 at 12:39

co.uk is in fact TLD

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I don't think you're right about that. As others have said, .uk is the TLD; Nominet (the UK namespace authority) makes this point fairly clear in nic.uk/registrants/register/choose . It is, however, a TLD in which direct registrations are not currently permitted; instead, a fairly-constant list of SLDs have been created, and direct registrations are allowed in these. –  MadHatter Oct 4 '10 at 16:29
    
See here as well dotuklaunch.co.uk –  Christian Seiler Feb 12 at 20:44

.co.uk is not a TLD, but .uk is a TLD and ccTLD (country code top level domain).

The manager of .uk have decided to divide its sub domains into commercial, education etc. But others has not, like .se or .us. That is up to the owner of the TLD to decide how to do.

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