Somewhere in the early 2000s, back when Borders still existed and Barnes & Noble had entire aisles devoted to O’Reilly books, I first picked up DNS and BIND. Not in a “bought it new and put it on the shelf” way, but in the “sitting cross-legged on the carpet for three hours until my leg goes numb” kind of way.

Back then I was just another young engineer orbiting around early web hosting companies—configuring Apache vhosts (long before NGINX was a thing), juggling IPs, and pretending to fully understand zone files. DNS was something you had to know, even if most people preferred not to.

DNS and BIND was one of those books you didn’t read cover to cover like a novel, but you did keep coming back to. It had real staying power. You could get through the first few chapters and suddenly understand what a subdomain really is—without guessing or oversimplifying. So, when people now ask me:

“What’s a subdomain?”

I default back to those early learnings. And honestly, that book still holds up.

Okay, So What Is a Subdomain?

In simplest terms, a subdomain is a domain that’s part of a larger domain. If example.com is the domain, then blog.example.com is a subdomain of it.

But that’s the browser-friendly version of the story.

In DNS terms, a subdomain is any domain that lives within another domain’s namespace. It doesn’t matter if it’s one label deep like blog.example.com or five like dev.stage.api.us-west.example.com—they’re all subdomains of example.com.

Even example.com is a subdomain—of com.
And com is a subdomain of the root.
And the root? That’s just . (which we all politely ignore most of the time).

It’s All About the Tree

DNS is structured like a tree. The root is at the top, and everything branches out from there. Each dot-separated label is just another level down.

.
└── com
    └── example
        ├── www
        └── blog

In that tree, blog.example.com is a subdomain. So is www.example.com. So is support.europe.internal.example.com.

The number of dots isn’t what defines a subdomain—position in the hierarchy is. If it lives beneath another domain in the DNS tree, it’s a subdomain.

Control Is the Real Divider

One of the key takeaways from DNS and BIND is that subdomains aren’t just about names. They’re about control.

In DNS, when you delegate a subdomain (say, using NS records to point app.example.com to a different set of name servers), you're handing over authority. That subdomain becomes its own little zone of power. A mini kingdom inside the larger realm of example.com.

This is how big services scale. It’s how enterprises isolate environments. It’s also how you end up with oddly specific configurations like vpn.prod.internal.london.office.example.com.

And yes, someone probably has to maintain that.

Subdomain vs. Subdirectory

This comes up a lot:

• blog.example.com → subdomain
• example.com/blog → subdirectory

The difference is where the decision-making happens.

Subdomains are routed via DNS. Your browser asks, “Where do I find blog.example.com?” and DNS replies with an IP. That means blog. could even be hosted on a completely different server, continent, or platform than example.com.

Subdirectories, on the other hand, are handled by the web server behind example.com. DNS never sees them. They’re just paths within the same server (or some load-balanced setup if you’re fancy).

Why It Still Matters

As someone who eventually started a small hosting business (years after those bookstore reading sessions), I can say this: understanding subdomains isn't just for theory. It's practical, daily-use knowledge.

Subdomains let you separate services cleanly, manage tenants in multi-tenant systems, hand off parts of your infrastructure, and avoid unnecessary coupling. They're also perfect when you want your status page to stay up even if everything else is broken.

And yes, they make weird product ideas like leasing subdomains possible. (Which is, in fact, a real thing I built.)

TL;DR – The DNS and BIND-Approved Definition

A subdomain is any domain that’s part of a broader domain’s namespace. It’s a label that sits somewhere beneath another in the DNS hierarchy, and it can be as simple or as deeply nested as needed.

It’s not about how long the name is or how many dots it has.
It’s about where it sits in the tree—and who controls it.

If you’re ever in doubt, find a copy of DNS and BIND—the 5th edition if you want that early-2000s bookstore nostalgia. It won’t just tell you what a subdomain is.

And if you ever want to monetize your own subdomain and/or lease one, check out Sublet.

It’ll tell you why it matters.