If you've been building a website and someone asked you "what's your SLD going to be?" — and you quietly panicked — you're not alone. Domain terminology is full of acronyms that sound more technical than they actually are.
Here's the good news: there are really only two things you need to know. Every domain name is made up of an SLD (second-level domain) and a TLD (top-level domain). Once you can spot them, you can decode any URL on the internet.
Breaking Down a Domain Name
Let's start with the simplest possible example: google.com. What parts make up that URL?
Reading right to left (which is how DNS actually works), you have:
- .com — the top-level domain (TLD). The extension.
- google — the second-level domain (SLD). The brand name.
- https:// — the protocol. Not part of the domain.
- /search — the path. Also not part of the domain.
The SLD and TLD together — google.com — are what you actually register and own. That pair is the domain name.
The TLD: Top-Level Domain
The TLD is whatever comes after the last dot. It's the highest level of the domain name system — which is why it's called "top-level," even though it appears at the end of the URL.
There are three main categories of TLDs:
Generic TLDs (gTLDs)
These are open to anyone and carry no country association. Classic examples: .com, .org, .net. Newer ones: .app, .shop, .tech, .xyz, and hundreds more.
Country Code TLDs (ccTLDs)
Two-letter extensions assigned to countries and territories: .uk for the United Kingdom, .de for Germany, .jp for Japan, .ca for Canada. Some are restricted to residents; others (like .co, .io, .ai) are open globally.
Sponsored TLDs (sTLDs)
Restricted extensions tied to a community or industry: .edu for accredited US universities, .gov for US government, .mil for the US military. You can't just register one of these — you have to qualify.
TLDs are managed at the top level by ICANN (the Internet Corporation for Assigned Names and Numbers). Each TLD has a designated registry — a company or organization that runs the back-end infrastructure. For .com, that registry is Verisign. For .shop, it's GMO Registry. For .ai, it's the government of Anguilla.
The SLD: Second-Level Domain
The SLD is the part right before the TLD. It's the part you choose — your brand, your name, your project. When you register a domain, you're really registering an SLD under a specific TLD.
A few examples to make it concrete:
| Full Domain | SLD | TLD |
|---|---|---|
| apple.com | apple | .com |
| openai.com | openai | .com |
| wikipedia.org | wikipedia | .org |
| stripe.com | stripe | .com |
| character.ai | character | .ai |
| github.io | github | .io |
The SLD is what makes your domain yours. Two businesses can't own the same SLD under the same TLD — that's the whole point. There's only one google.com, but google.net, google.co.uk, and google.app are technically different registrations (Google happens to own most of them defensively).
Subdomains and Third-Level Domains
Things get slightly more interesting when you look at URLs like mail.google.com or shop.nike.com. What's going on with that extra word at the front?
That extra word is a subdomain. Technically, it's a third-level domain. The structure goes:
Key thing to know: subdomains are free. Once you own yourbrand.com, you can create as many subdomains as you want — blog.yourbrand.com, shop.yourbrand.com, app.yourbrand.com, support.yourbrand.com. You don't register them separately; you just add a DNS record and they start working.
This is different from blogyourbrand.com, which would be a completely separate SLD that you'd need to register and pay for independently.
The Weird Case: Two-Part TLDs
Some countries use a two-level TLD structure: .co.uk, .com.au, .co.jp, .ac.uk. This is where it gets a bit confusing.
Take bbc.co.uk. Reading right to left:
.uk— the actual TLD (United Kingdom).co— a second-level indicator for "commercial"bbc— the brand name (technically a third-level domain here)
But colloquially, most people treat .co.uk as if it were a single TLD — and it functions that way for business purposes. When you register bbc.co.uk, you're essentially registering at what most registrars treat as the "SLD" slot (even though technically it's the third level).
Other countries with this pattern:
.co.uk/.ac.uk/.gov.uk— United Kingdom.com.au/.net.au— Australia.co.jp/.ac.jp— Japan.co.nz— New Zealand.com.br— Brazil
Why This Matters When You Register
Understanding SLD vs TLD helps you make better choices when registering a domain:
- Your SLD is your brand. It's what people remember, type, and share. Choose it carefully — you'll live with it.
- Your TLD is your context. .com says "mainstream business." .ai says "AI company." .shop says "e-commerce." .org says "organization." Pick the one that matches your positioning.
- You can own the same SLD under multiple TLDs. Many businesses register their brand across .com, .net, .org, and other TLDs defensively.
- Subdomains are free, extras are not. Before registering
shopyourbrand.com, consider just usingshop.yourbrand.com— same user experience, half the cost.
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To lock in the concept, here's how the same brand might look across different extensions:
| Full Domain | SLD | TLD | Context |
|---|---|---|---|
| acme.com | acme | .com | Global business |
| acme.ai | acme | .ai | AI company |
| acme.io | acme | .io | Developer tool |
| acme.shop | acme | .shop | E-commerce |
| acme.co.uk | acme | .co.uk | UK business |
| acme.de | acme | .de | Germany |
Same SLD, different TLDs, different contexts. Each combination is a separate domain that must be registered independently.
Quick Reference
A one-line summary you can use forever:
SLD = your brand. TLD = the extension. Put them together with a dot and you have a domain.
Everything else — subdomains, protocols, paths, TLD categories — builds on top of that simple idea.
Related Reading
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- Browse All 636 TLD Guides
Now You Know the Pieces — Pick Yours
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Find My Domain →Frequently Asked Questions
What is a top-level domain (TLD)?
A top-level domain is the part of a domain name that comes after the final dot — like .com, .org, .ai, or .shop. TLDs are managed by ICANN and operated by designated registries. They are the highest level in the domain name hierarchy, and every website has one.
Is .com a TLD or an SLD?
.com is a TLD (top-level domain). The part before .com — like 'google' in google.com — is the SLD (second-level domain). Together they form the registrable domain name.
What does the "co" in .co.uk mean?
In .co.uk, the "uk" is the TLD (the country code for the United Kingdom), and "co" is a second-level domain indicating a commercial entity. This is a two-level naming structure used in several countries — .co.uk for UK businesses, .ac.uk for UK academic institutions, .gov.uk for UK government. The actual brandable part of the name comes before .co.uk.
Can I register just a TLD by itself?
No. You cannot register just .com or .ai — these are managed by ICANN and operated by registries, not available to individuals or businesses. What you register is an SLD + TLD combination, like "yourbrand.com" or "yourbrand.ai". The TLD portion is only usable as part of a complete domain name.
What's the difference between a subdomain and an SLD?
The SLD is the main brand part of your domain (the "google" in google.com). A subdomain is something you create yourself that sits in front of your SLD, separated by a dot — like "mail.google.com" or "shop.yourbrand.com". Subdomains are free to create once you own the domain, and they behave like separate sites for many technical purposes.
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