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I have seen so many websites with two TLDs, like this one below:

example.com.ph (not real FQDN, by the way)

I am confused on whether what is the actual TLD server that the root server is redirecting me, in theory.

Like, is .ph the TLD or .com or both .com.ph?

If it's .ph, then SLD is the .com?

If .com is the TLD, then what is the .ph for?

I would understand better if the .com.ph is the TLD, but what should I call it? Combined TLD?

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The DNS is a tree, inverted if you want.

It starts with the root, aka ., typically written on top.

Then just below you have TOP level domains, aka TLDs. You can find the list of them at https://www.iana.org/domains/root/db

Do note they change, albeit slowly. There can be changes in countries, but most often they are changes in gTLDs, following ICANN 2012 round of new TLDs and supposedly some future new rounds.

ccTLDs are delegated to government. For your example: https://www.iana.org/domains/root/db/ph.html showing the Philippines owning it.

Each ccTLD (country) decides then how they handle their TLD, if it is flat, or if it is again subdivided. In history, some started subdivided (ex: uk had co.uk, me.uk, etc.) and then opened directly below TLD (you can now register directly under uk).

So in your example, in precise naming, ph is the TLD. com.ph is an 2LD or Second Level Domain.

It may be seen sometimes people will say com.ph is the TLD because they are thinking about the registration plane, not the resolution plane. The resolution plane is the DNS and the structure explained above: TLDs are single labels below root, then everything below is "Second Top" level (SLD or 2LD), then third and so on (you can look at us TLD for examples of names being 4th, 5th or even 6th levels below root). But, the registry of ph for example may have decided that you can only buy domains below com.ph not directly below ph. That is totally their right. As such some people will consider it is useless to speak about ph directly, because you can't "do" anything below it, you can only act below com.ph.

So it happens that TLD will often be used (misused) for labels that are technically not first level below root. Coming from the web, because of cookies, you can find sometimes also "eTLD" for "effective TLD", like "it is not technically a TLD because not one label below root, but act as one in the sense people register domains below it".

But the real proper term should be "public suffix". It means "anything" (TLD or not) that does exist in the public DNS, and is a valid suffix, that is a point (anchor) below which you can register names.

Also remember that registration can happen at multiple levels at the same time. Verisign handles .com TLD and you can register below it, but then Centralnic bought uk.com (from Verisign) and Centralnic acts as a registry so you can buy domains under uk.com "TLD". As such, both com and uk.com are valid public suffixes.

But going back to your example in com.ph, there com IS NOT a TLD. And any domain never has 2 TLDs, there is only one, technically the first label when starting from the right.

Terminology wise, the WHATWG Community (browsers basically) enacted those terms to be stricter at https://url.spec.whatwg.org/:

  • A host’s public suffix is the portion of a host which is included on the Public Suffix List.
  • A host’s registrable domain is a domain formed by the most specific public suffix, along with the domain label immediately preceding it, if any.

See also this RFC on DNS terminology if you want to be sure to exact proper technical terms in all cases: https://www.rfc-editor.org/rfc/rfc8499 but it is mostly dealing with the resolution plane, not the registration one. See its definition of TLD:

Top-Level Domain (TLD): A Top-Level Domain is a zone that is one layer below the root, such as "com" or "jp". There is nothing special, from the point of view of the DNS, about TLDs. Most of them are also delegation-centric zones (defined in Section 7), and there are significant policy issues around their operation. TLDs are often divided into sub-groups such as Country Code Top-Level Domains (ccTLDs), Generic Top-Level Domains (gTLDs), and others; the division is a matter of policy and beyond the scope of this document.

You can also view current list of public suffixes at https://publicsuffix.org/ ; do remember this is a manually curated list, which obviously includes all TLDs, but much more because of what we discussed above.

PS: the root servers are not "redirecting" you, there is no concept of "redirection" in DNS world, this was invented 15 years after DNS was invented, when HTTP was created.

You can ask root servers about ph and they reply it is delegated to:

$ dig +noall +auth ph @a.root-servers.net NS
ph.         2d IN NS ns4.apnic.net.
ph.         2d IN NS ns2.cuhk.edu.hk.
ph.         2d IN NS 1.ns.ph.
ph.         2d IN NS ph.communitydns.net.

You can then ask any of these nameservers about where com.ph is delegated, if it is (a "dot" in a name does not technically imply always a delegation; for example gouv.fr right now is not technically delegated out of fr even if it works perfectly):

$ dig +noall +auth com.ph @ns4.apnic.net. NS
ph.         15m IN SOA ph-tld-ns.dot.ph. sysadmin.domains.ph. (
                2023010421 ; serial
                21600      ; refresh (6 hours)
                3600       ; retry (1 hour)
                2592000    ; expire (4 weeks 2 days)
                86400      ; minimum (1 day)
                )

So by this reply you can see, com.ph not being technically delegated from ph, which means it is handled by the same nameservers and hence the same organization (you would get the opposite result if trying with com and uk.com because as explained above they are handled by different organizations).

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  • FWIW .au also recently added direct registrations e.g. example.au
    – Steve
    Commented Jan 4, 2023 at 21:12
  • Wait, so, if I can understand you correctly, mydomain.com is the SLD and .ph is the TLD? Commented Jan 5, 2023 at 2:40
  • The DNS is a tree, with levels. Each new label adds a new level. . is "0" label so it is the root. example. is one label (below root) so it is the TLD. foobar.example. has 2 labels so it is a second level domain name, acme.foobar.example. has 3 labels so it is third level and so on. But at the end of the day, where and why does it matter? I mean besides setting things right with proper terminology, which is important, you then discover lots of approximation and misuses (like TLD used as synonym for valid suffix, etc.). Commented Jan 5, 2023 at 3:13
  • Okay, I get the point. I am just confused about like if what you said is really the case for most is that example would be the subdomain, while SLD would be the .com and TLD is .ph. I am very curious about the proper terminology as I will be teaching this and wanted to be prepared if they ask this question. Commented Jan 5, 2023 at 4:47
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    Technically Top level Domain is for the first label below root. Anything further below is not anymore a TLD, but this approximation is often done instead of saying "valid public suffix". I recommend you take some time to read RFC 8499 as it is really a corpus of definitions. Do remember vividly though that it deals mostly with the resolution phase, what happens at the DNS side of things, and not so much at the registration phase, when with have the question of "where - under which "TLD" - can I register a domain name". Commented Jan 5, 2023 at 13:34

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