How many domain levels are there?

Question

.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?

Answer 1

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.

Answer 2

I just jumped onto my DNS and created this:

_{(source: 41085.org)}

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

Answer 3

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

Answer 4

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.