site.com is 1 period
site.co.uk is 2 periods
alpine.k12.ut.us is 3 periods?
Is k12.ut.us the TLD? Or is k12 the domain name? How many periods can a TLD have?
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Zero periods, or one if you include the optional one to the right of the TLD (com.). The TLD is the rightmost portion of the domain name. In your example, that would be
k12 would be a third level domain name, though I don't think I've seen that used.
k12.ut.us is a subdomain of
Wikipedia has a brief writeup of the different types of TLDs.
You can look for RFC documments when you want definitions and answers about Internet and domains. You find all RFC:s at http://www.rfc-editor.org/
RFC 1591 defines DNS (Domain Name System), TLD (Top Level Domain name), and ccTLD (country code Top Level Domain name). Those are the name after the first dot, that you normaly don't use. So "serverfault.com" is actually "serverfault.com.". The full name of this site is "www.serverfault.com." and has TLD "COM". The domain name "bbc.co.uk" has the ccTLD (which also is a TLD) "UK".
RFC 3071 talks about TLD:s and ccTLD:s as defined in RFC 1591, so it would be good to have a look into. You might want to search "IDNA" and "Punycode" to :-)
Also notice that any name in DNS can have different resources connected to it. So bbc.co.uk could be a machine and/or a subdomain with www.bbc.co.uk
The TLD (top-level domain) is the very last part of the domain name. For example, in
.us is the TLD and everything below it are levels of sub-domains or hosts on a sub-domain. Even for
.uk is the top-level domain. The powers that manage
.uk decided at some point to offer domains as sub-domains of
co.uk instead of registering domains directly on
.uk like you can with some, like