Till today, I thought I knew the definition of TLD, SLD, apex domains and so on. But speaking with some people it seems that I'm making confusion. Here's what I think:

  • .com is a top level domain (TLD), aka first level domain;
  • stackexchange.com is a second level domain (SLD) and an apex domain;
  • .co.uk is a top level domain aka first level domain;
  • google.co.uk is a second level domain and an apex domain.

And now the hard part: .name domains. As you know, you are free to register either the domain surname.name or the domain name.surname.name. If you register name.surname.name, then surname.name becomes unavailable for registration.

Supposing that I have registered only name.surname.name and not surname.name:

  • name.surname.name is a third-level domain and an apex domain.

Here's the question: am I right and, if not, what mistakes am I making?

  • 1
    Just an FYI, Nominet (the UK registry) has made it possible in the last year to register domains directly at the .uk TLD. Commented Sep 15, 2015 at 15:28
  • Supposing that I have registered only name.surname.name and not surname.name: You can never do that. Unless you own surname.name or that domain allows you to register its sub domain you cant do that. Commented Sep 16, 2015 at 7:36
  • 1
    @Hanky .name are unique in this respect (see duskwuff's comment below). In fact, when .name domains were first launched, you could only register third level domains. However, the registrars I've tried recently only seem to allow SLD registrations. Reference: en.wikipedia.org/wiki/.name
    – MrWhite
    Commented Sep 18, 2015 at 13:02

5 Answers 5


Just count the dots...

.co.uk is a top level domain aka first level domain;

uk is a TLD. co is a SLD.
(Reference: http://www.nominet.uk/uk-domain-names/about-domain-names/uk-domain-family)

Although it's common to see .co.uk described as a ccTLD, it's really only the uk part that is the TLD.

google.co.uk is a second level domain and an apex domain;

google is a third level domain.

And now the hard part: .name domains

...Just count the dots.

  • 2
    and yet .co.uk is directly equivalent to .com. There are technical and practical reasons to argue these definitions either way, the truth is it doesn't actually matter. The various rules don't apply consistently to any one definition of these terms.
    – JamesRyan
    Commented Sep 15, 2015 at 9:48
  • 6
    @JamesRyan What is your source for saying they are equivalent? They might be treated equivalent in usage, but a simple example: co.uk has a whois entry, which is not the case for any TLD. So co.uk is NOT equivalent to .com would be my conclusion
    – Nanne
    Commented Sep 15, 2015 at 10:30
  • 2
    @nanne actually co.uk and uk both have whois entries nominet.uk/whois/lookup?query=co.uk nominet.uk/whois/lookup?query=uk
    – JamesRyan
    Commented Sep 15, 2015 at 10:39
  • 3
    The easiest way to highlight that .co.uk is a special case is to compare it to uk.com. co.uk is managed and domains sold by nominet as if it were a TLD, uk.com is simply a domain with subdomains resold by a private company. Different protections and legal ownership status of the subdomains of each apply. So in strictest terms you could call the very last part the TLD, but when generalising even in a purely technical way it isn't that clearcut. The intended meaning in any instance will depend on context.
    – JamesRyan
    Commented Sep 15, 2015 at 10:53
  • 1
    @JamesRyan and the TLD .google is also owned by a private company (though it seems that they are not selling domains), being a TLD. Nothing being proved here. Commented Sep 15, 2015 at 19:01

You seem to confuse the technical definition with registration policies.

.co.uk is not a TLD. .uk is a TLD. As w3d notes, "just count the dots".

But .co.uk is "special" in the way that you may register your domain name under this second-level domain. It’s an effective top-level domain (eTLD).

Mozilla hosts a list of these eTLDs, which they call Public Suffix List. They define a public suffix like this:

A "public suffix" is one under which Internet users can (or historically could) directly register names. Some examples of public suffixes are .com, .co.uk and pvt.k12.ma.us. The Public Suffix List is a list of all known public suffixes.


According to https://publicsuffix.org/list/public_suffix_list.dat .name has second level domains that are considered top level domains, but there's no list of them. You'll have to research each name individually to determine if it's a valid, owned domain name, or a top level domain name with owned domains below it.

See Get the subdomain from a URL for more information on what TLDs are, and how to find subdomains.

  • 2
    If it makes you feel any better: .name is unique and bizarre in allowing both second-level and third-level registrations, and this design decision is generally considered to have been a huge mistake. (As seen in the Public Suffix List, it causes some serious complications for software.) In practice, third-level .name domains are rarely used.
    – user8879
    Commented Sep 15, 2015 at 23:54

I think it is good to standardize on the terminology explained at https://url.spec.whatwg.org/#host-miscellaneous

In short:

A host’s public suffix is the portion of a host which is included on the Public Suffix List.

So from your examples list, today (2021), both com, co.uk AND uk are public suffixes.

Historically this was also called "effective TLD" or "eTLD" because, as other answers already wrote about, technically a "TLD" is only the last label, the right most one, after the last dot. So uk is a TLD but technically co.uk is not, but is considered to be an "effective TLD" because it works "as if" it is a TLD, regarding registration rules. But this is still ambiguous, hence "public suffix" is far better than "effective TLD".

Also, while we are here, ban absolutely "extension", even if that misuse is widely shared. Domains do not have extensions. A dot, in a domain name, has a very specific and completely different meaning than a dot in a filename, and especially for the concept of extension that was introduced by some OS.

A host’s registrable domain is a domain formed by the most specific public suffix, along with the domain label immediately preceding it,

So stackexchange.com and google.co.uk are registrable domains (and are in fact registered).

Again, historically, once we had "eTLD", we could say that those "registrable domain" are "eTLD + 1", +1 meaning +1 label on the left. That terminology was often used by browsers, specifically to define security policies around which cookies can be shared between sites.

The link gives a pseudo algorithm to apply in both cases to define these two items. It has also a table with examples that I reproduce here:

Host input                                 |Public suffix |Registrable domain
com                                         com            null
example.com                                 com            example.com
www.example.com                             com            example.com
sub.www.example.com                         com            example.com
EXAMPLE.COM                                 com            example.com
github.io                                   github.io      null
whatwg.github.io                            github.io      whatwg.github.io
إختبار                                         xn--kgbechtv   null
example.إختبار                                 xn--kgbechtv   example.xn--kgbechtv
sub.example.إختبار                             xn--kgbechtv   example.xn--kgbechtv
[2001:0db8:85a3:0000:0000:8a2e:0370:7334]   null           null 

Remember however that the publix suffix list is not something baked up in any protocol like DNS and available in a decentralized manner. It is a manual list maintained by volunteers. Which means you can't access it without downloading it (finding zone cuts in the DNS can give you a similar information but not the same: gouv.fr is a separate public suffix from fr but there is no delegation points between the two), and you have no guarantee it is either exhaustive or up to date.

Also for everything DNS related the current good document to keep around is RFC 8499 "DNS Terminology". It is however more tailored for the DNS operations side, that is how names resolved, instead of the DNS registration side, that is how names are registered and where. It does standardizes what a "domain name" is and what a "TLD" is.

It could be useful to consult to straighten out your use of "apex" in:

name.surname.name is a third-level domain and an apex domain.

That name, or any other name in fact, is NOT an "apex" domain. "apex" is not an adjective you can put at some domains and not others. All domains have an apex, it is a specific part of the tree, relative to each node. As said in above RFC:

Apex: The point in the tree at an owner of an SOA and corresponding authoritative NS RRset. This is also called the "zone apex". [RFC4033] defines it as "the name at the child's side of a zone cut". The "apex" can usefully be thought of as a data-theoretic description of a tree structure, and "origin" is the name of the same concept when it is implemented in zone files. The distinction is not always maintained in use, however, and one can find uses that conflict subtly with this definition. [RFC1034] uses the term "top node of the zone" as a synonym of "apex", but that term is not widely used. These days, the first sense of "origin" (above) and "apex" are often used interchangeably.


Top-Level Domain (TLD)

Every domain name is composed of two parts: a TLD (top-level domain) and a second-level domain. In the domain name example.com, the .com part is the top-level domain, and the word example is the second-level domain. While there are an almost infinite number of second-level domains, there are a limited number of generic TLDs, see examples below.


.com Not restricted. This is the original TLD for businesses, but it has been used by many non-business groups.

.biz Restricted for use by businesses; .biz was added because .com is being used by some groups and individuals that are not businesses.

.name Restricted to use by individuals who want to register their own name as a domain name; this allows people to have their own personal Web sites without using .com or other TLDs.

.org Not restricted. Intended for use by organizations such as non-profits.

.net Not restricted. Intended for use by organizations who contribute to the construction or maintenance of the Internet.

In addition to generic TLDs, there are a growing number of country code (cc) TLDs which associate Web sites with a particular country. For instance, .us can be used by companies in the United States . If you are an international company or do extensive business with another country, consider registering a domain name with the appropriate country code.


.co.uk The United Kingdom

.co.za South Africa

Side Note ( Just a FYI )

There is not much of a difference SEO wise when it comes to .com vs .net vs .org domains because all of them are commonly used general TLDs.

The only effect of TLD on SEO is, if your TLD is for particular region, you may not show up as a search results in other geo region.

Google expands its general TLDs and it is being updated regularly. Read Here Geotargetable domains

As an example if you have a .in domain, your ranking in Google India may be quite high when compared to Google Japan.

  • 2
    Conclusion There is not much of a difference SEO wise when it comes to .com vs .net vs .org domains because all of them are commonly used general TLDs. I think the question is not about TLDs comparison in SEO perspective. Commented Sep 15, 2015 at 15:31
  • I just elaborated on that too a bit, becuase generally the next question to follow is almost always how does it affect SEO. I hope it is okey that I added it there as I feel it offers a great addition to anyone reading this question. Sort of a two birds with one stone :D Commented Sep 15, 2015 at 15:32
  • My question is only about terminology. Nothing more than that :)
    – hey hey
    Commented Sep 15, 2015 at 17:46
  • About the first sentence, there are also domain names made from more than two parts, like www.math.hu-berlin.de (consisting of 4 parts). Commented Sep 15, 2015 at 19:03
  • 1
    @heyhey My answer where aimed to assist not only you, but the public reading your post as well. I don't think it is wrong of me to have added the SEO part in there as it will be asked I can guarantee it, but if it makes you happy I will edit my answer and if it helps you, so some love with a upvote please (: Commented Sep 15, 2015 at 19:42

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