When two words are very similar, it’s easy to get confused. Which witch is which? Whether, weather, and wether. Affect vs effect… Okay, you know, English sucks. We have way too many words that will drive you to drink, and if you know anyone who’s learned English as a second language, please take time to tell them how amazing they are. My father’s wife is Japanese, bilingual in French, and is learning English. I know a smattering of French. Our conversations are fantastically amusing and thankfully we have great senses of humor.

Because I’m that familiar with the crazy of my native language, I have no surprise that people get subdomains and domains confused. Here’s the basic statement:

A subdomain is not the same as a domain.

That’s it. But since I don’t expect everyone to know what the heck I just said, I’m going to explain. Remember, don’t think you’re stupid for not knowing this! You can’t magically know everything, you have to learn it, and there are people like me who want to help you. Where the confusion kicks in isn’t that we call it a ‘subdomain’ but that the official definition is “a subdomain is a domain that is part of a larger domain.” So we’ve just said a domain of a domain, and yet here I am pushing you and saying that a subdomain isn’t a domain when it clearly is.

It is and it isn’t.

• A domain is pretty simple: elftest.net is a domain. It’s the solid basis that all websites are built on.
• A subdomain is a subset of the domain: tools.elftest.net is a subdomain on elftest.net.

Notice how ‘tools.’ is in front of elftest.net? That extra period between tools and elftest is how we know this is a sub domain. The .net part is called the ‘Top Level Domain’ and any time you see www, that actually isn’t a subdomain, but a special term… You know what, let’s break this down with a picture.

You can ignore protocol for now (we can get into that another time). What we’re looking at is this:

URL: http://www.example.com/index.html
Top-level domain name: com
Second-level domain name: example.com
Host name: www.example.com OR example.com

Why is www special? It has to do with a lot of boring history, but suffice to say that used to be how we knew it was a webpage! Now we use http:// to say ‘This will be a webpage’ so many of us (myself included) feel that www is unnecessary and just makes URLs longer. However because of history, http://www.elftest.net and http://elftest.net will forever point you to the same place. This actually means that www is a subdomain, but it’s a very special one that points to the same place as no www at all. In very rare cases, a fancy website will redirect www and non-www to different places, but this is the exception, not the rule. Good SEO practices are to have the www and non-www point to the exact same place.

The meat of the matter is that most of the time, when someone asks ‘What’s your domain?’ they really mean ‘What’s your host name?’ My host name is elftest.net (or ipstenu.org or halfelf.org…. I have a lot of domains

Now let’s look at a subdomain.

They look shockingly similar, except that instead of www in front, I have sub. So what’s the deal here? Well because I’m using something other than www, I’ve designated sub.example.com as a subset of example.com, and thus a subdomain. Yes, it’s backwards. Sub should be below or behind, but remember, we’re calling .com the top level domain, so right-to-left this makes more sense.

I know. It’s all clear as mud. Even writing this I sat there and muttered “This stuff is nuts.” I know all this didn’t explain everything as clearly as I could wish, but I’ll break it down into the simplest terms that, while not 100% technically accurate, will tell every decent web tech what you mean:

When someone asks “What’s the subdomain?” you answer ‘sub.example.com’

If someone asks “What’s the domain?’ you say ‘example.com’ (sometimes they’ll ask “What’s the main domain?”)

If you’re on Multisite and someone asks “What’s the mapped domain, and what subdomain does it point to?” you say ‘mappeddomain.com and it points to mapped.example.com’

And never ever use domain mapping plugins for your subdomains. Those are for grownup domains only, not your subdomains.

For extra credit: Third level domains are what you get when you see things like example.co.uk – example isn’t a subdomain here, it’s the main domain. co.uk is the TLD. Why third? Well, we’d already used sub and second, and we needed some way to say that this is part of the primary URL, and not a subset. Also geeks love to confuse people.