A domain name is hierarchical and is read right to left, from most general to most specific. The levels are the root (an unnamed zone shown as a trailing dot), the top-level domain (TLD, e.g. .com), the second-level domain (e.g. example), and any number of third-level and deeper labels — the subdomains you control yourself.
Take a single address apart and the layers appear in order. In blog.example.com, the .com is the top level, example is the second level, and blog is the third level. Each step to the left is a more specific child of the label on its right, which is exactly why domains are read from the end backwards.
This page covers the full stack of levels. If you only need the contrast between the last two tiers, see second-level vs top-level domain; for how a resolver walks these tiers, see how the DNS hierarchy works.
What is each level, from the top down?
The Domain Name System is a tree. At its summit is a single unnamed point, and every label below it adds a level. The table lists the levels in resolution order, from the root downward.
| Level | Name | Example | Who controls it |
|---|---|---|---|
| 0 | Root | . (trailing dot) | IANA / ICANN, via the root zone |
| 1 | Top-level domain (TLD) | .com | The TLD registry (e.g. Verisign) |
| 2 | Second-level domain (SLD) | example | You, via your registrar — usually the part you register |
| 3 | Third-level domain (subdomain) | blog | You — created freely once you own the domain |
You can keep going. A fourth level (uk.blog.example.com) and beyond are all just more subdomains nested under the level to their right. The technical ceiling is generous: a name may span up to 127 levels and 253 characters in total, with each individual label capped at 63 characters.
A labelled tier of the DNS naming tree. Each dot separates one level from the next; levels are numbered from the root (level 0) outward, so the TLD is level 1, the second-level domain is level 2, and subdomains begin at level 3.
What is the root, and why the trailing dot?
Above every TLD sits the root — a single zone with no name of its own. It is written as a bare dot at the very end of a name: example.com. You almost never type that dot, because resolvers add it for you, but it is always implied. The root is the starting point of every DNS lookup: a resolver asks the root which servers handle .com, then asks those for example.com, and so on down the tree.
Fully qualified vs. relative
A name that includes the trailing dot, like www.example.com., is a fully qualified domain name (FQDN) — it names every level up to the root and is therefore unambiguous. A name without the dot may be treated as relative and have a search suffix appended by the local system.
Second level vs third level: where do you fit in?
The two levels most people interact with are the second and third. The second-level domain is the label immediately left of the TLD — example in example.com — and it is normally the part you pay to register. Once that registrable domain is yours, you are free to create third-level domains (subdomains) such as www, blog or shop without registering anything new; you simply add DNS records.
- Second-level domain (SLD): usually what you register; combined with the TLD it makes the registrable domain (e.g.
example.com). - Third-level domain: a subdomain you create and point wherever you like — a blog, a store, a staging server.
- Deeper levels: fourth level and beyond are just subdomains of subdomains, useful for large or regional structures.
The registrable label is not always level 2
Under TLDs that use a public registration suffix, the second level is reserved. In example.co.uk, the co is a second-level domain operated for registrations, so the name the public actually buys (example.co.uk) is technically a third-level domain. “Registrable domain” and “second level” are not always the same thing.
Why are domain names read right to left?
It mirrors how the system resolves. Because the root is on the right (as the silent dot) and specificity increases to the left, scanning a name from right to left follows the exact path a resolver takes: root → TLD → second level → subdomain. So in mail.eu.example.com, reading backwards gives you root, then .com, then example, then the eu region, then the mail host — each label a child of the one before it. The structure and the rules that govern it are defined in RFC 1034 and RFC 1035.
★ Key takeaways
- Domain names are layered and read right to left: root → TLD → second level → third level (subdomain).
- The root is the unnamed top, shown as a trailing dot; every lookup starts there.
- The second-level domain is usually what you register; third-level labels are subdomains you control for free.
- With suffixes like
co.uk, the registrable name is a third-level domain, not a second-level one. - Names can reach 127 levels / 253 characters; an FQDN spells out every level to the root.
Frequently asked questions
What are the levels of a domain name?
Reading right to left: the root (an unnamed zone shown as a trailing dot), the top-level domain or TLD (.com), the second-level domain (example), and one or more third-level domains, also called subdomains (www or blog). You can nest further levels below the third if needed.
What is a third-level domain name?
A third-level domain is the label placed to the left of the second-level domain, such as blog in blog.example.com. In practice it is a subdomain that you create and control yourself once you own the registrable domain, so no extra registration is required.
What is a secondary (second-level) domain?
The second-level domain is the label directly to the left of the TLD, for example example in example.com. It is usually the part you actually register, and together with the TLD it forms the registrable domain.
Why are domain names read from right to left?
The Domain Name System is hierarchical and resolves from the most general level to the most specific. The right-most label (the TLD) sits closest to the root, so reading right to left follows the same path a DNS resolver takes when it looks a name up.
Is the registrable domain always the second level?
No. Under TLDs that use a public registration suffix, such as co.uk, the co label is itself a second-level domain operated for registrations, so what the public registers (example.co.uk) is technically a third-level domain. The registrable label is not always at the second level.
What is a fully qualified domain name (FQDN)?
A fully qualified domain name lists every label up to and including the root, ending in a trailing dot, such as www.example.com. It is unambiguous because it specifies the complete path through the hierarchy with no missing levels.
Sources & further reading
- RFC 1034 — Domain Names: Concepts and Facilities (defines the hierarchy)
- RFC 1035 — Domain Names: Implementation and Specification (label and length limits)
- IANA — Root Zone Management (the level above all TLDs)
- Related: second-level vs top-level domain, what is a subdomain, fully qualified domain name