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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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  • co.uk is in fact TLD
    – vps
    Feb 6, 2010 at 11:45
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    .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.
    – Anders
    Oct 4, 2010 at 16:24

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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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I just jumped onto my DNS and created this:

alt text
(source: 41085.org)

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, 2010 at 12:39
  • That is actually 127, as you have the top level "." too. The one above is and gd.
    – Anders
    Jan 2, 2015 at 3:40
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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, 2010 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, 2010 at 3:32
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    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, 2010 at 4:25
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    Who is the "owner" of a subdomain is decided by the one owning the domain above. So who owns sub1.google.com is decided by google.com and who owns news.bbs.co.uk is decided by bbs.co.uk. And who owns uk is decided by the owner of ..
    – Anders
    Jan 2, 2015 at 3:43
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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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