The top-level domain (TLD) is the ending of a web address — the part after the final dot, like .com. The second-level domain (SLD) is the label directly to its left — the unique name you register, like example. Put together, example + .com form the registered domain example.com.
Every domain name is a stack of labels separated by dots, and the internet reads them from right to left, moving from the most general level to the most specific. The right-most label is the top-level domain; the one immediately beside it is the second-level domain. The difference between them is partly position and partly who decides what they are.
If you have not yet read what a TLD is, that guide covers the top-level half in detail. This one zooms in on the relationship between the top level and the level just below it — the part you actually get to choose.
Which part is which?
The cleanest way to see the difference is to take a plain domain apart. Consider example.com:
- The top-level domain is
.com. You do not invent it; you pick it from a fixed list of extensions coordinated by ICANN and IANA. - The second-level domain is
example. This is the bit you make up and register, as long as nobody has taken it under that TLD.
So the TLD answers “which family of domains is this?” while the SLD answers “which specific name within that family?” You register the two as a pair: you cannot buy example on its own, only example.com, example.org, example.us and so on.
Second-level domain — the registrable label sitting directly to the left of the top-level domain. In example.com the SLD is example; combined with the TLD it forms the registered domain.
How do the levels stack up in a full address?
Real web addresses can have more layers than a bare two-part domain. The table below breaks down a simple .com name and a longer British example so you can see each level side by side.
| Level | In example.com | In shop.example.co.uk |
|---|---|---|
| Root | . (silent trailing dot) | . (silent trailing dot) |
| Top-level domain | .com | .uk |
| Second-level domain | example | co |
| Third-level domain | — | example (the registered name) |
| Subdomain | www (optional prefix) | shop |
The British column is the interesting one. Under .uk, the label co occupies the second level, and the name a business registers — example — actually sits at the third level. Yet most people would call example their “domain name,” because it is the part they chose and paid for. Position and everyday meaning do not always match.
Effective TLDs and the public suffix list
Combinations like .co.uk, .com.au and .gov.uk are sometimes called effective TLDs or public suffixes: registrants buy names directly beneath them, so for practical purposes they behave like an extension. Browsers and software use a maintained “public suffix list” to know where the registrable part begins, which is how they tell example.co.uk apart from a genuine subdomain. The true top-level domain, though, is still only .uk.
Why does the distinction matter?
Getting the layers right affects a few practical things:
- Branding. Your second-level domain is your name — the memorable word people type and share. The TLD frames it but the SLD carries your identity.
- Availability. The same SLD can be free in one TLD and taken in another, which is why
example.netmight be available whenexample.comis gone. - Subdomains. Once you own a registered domain, you can create subdomains like
blog.orshop.for free, because they live inside your domain rather than being registered separately.
A quick test
Find the last dot: everything to its right is the TLD. The single label just to the left of that dot is the second-level domain. Anything further left is either part of a multi-level suffix (like co.uk) or a subdomain you control. For the underlying tree structure, see how the DNS hierarchy works.
What do you actually register?
When you buy a domain, you are registering a second-level domain under a chosen top-level domain — the pair, not either half alone. Your registrar checks that the combination is free, then records you as the holder. From that point you own the whole registered domain and can build subdomains beneath it at will. If you are ready to do this, our walkthrough on how to register a domain name covers each step, and how to choose a domain extension helps you pick the TLD half.
★ Key takeaways
- The TLD is the ending (
.com); the SLD is the name just before it (example). - You pick the TLD from a fixed list but invent the second-level domain — and you register them as a pair.
- In multi-part suffixes like
.co.uk,cois the second level and your name sits at the third level. - Subdomains (like
blog.) sit left of the SLD and are yours to create for free once you own the domain.
Frequently asked questions
What is the difference between a second-level and a top-level domain?
The top-level domain (TLD) is the ending of a web address, after the final dot, such as .com or .uk. The second-level domain (SLD) is the label immediately to its left, which is the unique name you register, such as example in example.com. The TLD is chosen from a fixed list of extensions; the SLD is the part you invent.
What is a second-level domain example?
In example.com, the second-level domain is example. In wikipedia.org, it is wikipedia. In bbc.co.uk the situation is special: co is technically the second level under .uk, and bbc sits at the third level, even though bbc is the part the BBC registered and treats as its name.
Is the second-level domain the same as the domain name?
Not quite. People often call the whole thing the “domain name,” but technically the registered domain is the second-level domain plus the top-level domain together, such as example.com. The second-level domain on its own is just the example part. You register the SLD, but you always register it under a specific TLD.
What about co.uk — is that a TLD?
No. .uk is the top-level domain, and co is a second-level label offered under it. When you register something like example.co.uk, your chosen name example is actually at the third level. Combinations like .co.uk are sometimes called effective TLDs or public suffixes because you register directly beneath them, but the true TLD is still just .uk.
Where does a subdomain fit in?
A subdomain sits to the left of the second-level domain and is something you control once you own the domain. In blog.example.com, blog is a subdomain, example is the second-level domain, and .com is the top-level domain. You do not register subdomains separately; you create them yourself within a domain you already hold.
Sources & further reading
- IANA — Root Zone Database (the top-level domains you register beneath)
- ICANN — Glossary of domain terms
- IANA — Technical requirements for domains
- Related: what a TLD is, how the DNS hierarchy works, how to register a domain name