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One of the most frequent questions our tech team hears is, “Exactly what is a subdomain?” An important question, to be sure. The more pressing followup, though, should be, “When is the best time to use them?”

Subdomains are integral to the architectures of the sites that use them, as they signal key information to Google, such as:

• How the content and links on a subdomain relate to the content and links on the top domain.
• How search crawlers should decipher and index the information they find.
• How important the subdomain is to other portions of the site or even other subdomains.

Subdomain SEO is thus its own discipline within digital marketing and web development.

What Is a Subdomain?

A subdomain is a division of your website that you want to distinguish with its own unique identity and content. For instance, if Brafton wanted to create a subdomain for our blog page (we don’t), our subdomain name would be blog.brafton.com.

Your main domain — also known as a primary domain or a root domain — is essentially the name of your website.

In Brafton’s case, our main domain name is brafton.com. Notice I didn’t say www.brafton.com or https://www.brafton.com, which are technically our site URLs.

Why separate certain areas of your website into subdomains? Let’s take a look at an example of a site hierarchy:

How Subdomains Are Used

That diagram is pretty clear-cut and easy to follow, right? But let’s say instead of a simple FAQ page, you want to build out an extensive research library filled with white papers, datasheets, training materials, user guides and product operation instructions. All of a sudden, that nice, neat sitemap would get very complicated, very fast.

A subdomain lets you separate portions of your site that are extensive enough to warrant their own dedicated hierarchy without going through all of the trouble of setting up a new site with a new domain or confusing visitors with a completely different root domain.

Many companies use subdomains in this fashion, even Google itself. If you need assistance with any of the tech giant’s apps, you’ll be directed to this page:

https://support.google.com/

Notice that it’s not https://google.com/support, but https://support.google.com.

What Are the SEO Implications of Subdomains?

“So, it’s a good organizational tool,” you might be asking yourself, “but what does a subdomain have to do with SEO and organic rankings?” Excellent question.

First of all, there’s really no such thing as organization for organization’s sake when it comes to site structure and hierarchy. A website that’s confusing to navigate due to poor structure is going to inevitably be penalized by search engines.

A bad site layout will lead to visitors spending less time on your page or more time on the page if it takes them a long time to find what they’re looking for, resulting in a less than satisfactory experience and potentially hurting your search performance. By using subdomains to streamline your site hierarchy, you help users find the information they seek more efficiently, improving your website’s SEO performance.

If you’re keyed into the SEO-focused blogosphere, at this point, you’re probably thinking to yourself, “Wait, I thought subdomains were bad for SEO?”

There has been a lot of talk out there about the potential negative impact that subdomains can have on SEO performance, specifically that search engines penalize them or have difficulty parsing between main domains and subdomains.

The reasoning goes that Google’s algorithms will recognize your subdomains as sites separate from your main domain, and rank them individually. They’ll essentially be stranded on little SEO islands, and your website won’t benefit from any positive impact your subdomain would otherwise provide if it were a subdirectory instead.

Although the debate on subdomains’ SEO merits — or lack thereof — rages on, there’s reason to believe that previous warnings of “organic search cannibalization” may have been a tad overstated.

In an August 2016 Hangout session, Google Webmaster Trends Analyst John Mueller addressed the question head-on, stating that subdomains generally don’t hurt site rankings:

“[W]e recognize that some sites use subdomains as different parts of the site,” he said. “And, in the same way that other sites might use subdirectories.”

According to Mueller, Google’s algorithms are pretty adept at crawling subdomains and subdirectories equally well and making sense of it all.

That being said, he did caution against the use of wildcard subdomains designed to redirect anyone trying to access non-existent subdomains to a specific folder. Google’s algorithms have historically had difficulty crawling those specific kinds of subdomains.

What Is a Subfolder or Subdirectory?

A subfolder, also known as a subdirectory, is exactly as the name implies: a folder of pages that still exist and nest under the root domain. It is not viewed as a standalone site within Google’s eyes like a subdomain is.

The graphic below breaks down the distinctions between subdomain and subfolder for easy reference:

Site hierarchy best practices stress the importance of simplicity, recommending that organizations minimize the number of categories and subcategories included in any navigation sitemap. Kissmetrics, for instance, suggests keeping the number of site categories limited to anywhere from two to seven.

Second-level Domains Are Basically the Same as a Subdomain, Right?

Despite the nomenclature, the second-level domain is arguably the most important part of your main site name. That’s because top-level and second-level domains actually refer to the hierarchy of web addresses, not how they relate to your main website.

Again, take brafton.com. In this case, “.com” is the top-level domain (TLD), indicating that the site is a commercial website. Web address suffixes like .org, .edu and .gov are all top-level domains. Your domain TLD could also reference the site’s country: .ca, .it, .in, .uk, etc. We’re really talking about the building blocks of websites here.

With that in mind, “brafton” is the second-level domain. And really, that’s what people remember and associate with your main site. No one calls Google “google.com” or LinkedIn “linkedin.com.” That would be crazy.

How Do You Set Up a Subdomain?

There are basically two ways to create a subdomain: using a CNAME record or an A record. CNAME is a record that resolves to an alternative URL, while an A record resolves to an IP address.

Regardless of which approach you take, the record needs to be created according to your subdomain’s hosting specifications. If you don’t follow those hosting specifications, you run the risk of setting up a DNS record that won’t properly resolve when the hosting account receives requests from it. With that in mind, it could be a good idea to consult your domain provider (aka, your web hosting provider, or more simply, “domain host”) to get all the hosting information you’ll need and remove any guesswork from the equation.

How does all of this look in action? Let’s say you’re hosting through WP Engine and want to set up a dedicated website subdomain for your blog. You’ll need to go through an online portal that looks something like this:

When you go to create a subdomain, WP Engine will issue you a temporary URL to use. From there, you can choose to create either a CNAME record to resolve back to that URL or an A record to resolve your new blog to a specific IP address.

Finally, you’ll need to register your new subdomain with WP Engine so it knows how to properly resolve it.

When Does It Make Sense to Use Subdomains?

Given how extensive Google’s support page is, it’s a perfect candidate for subdomain status: There are a dozen apps to cover, each with unique troubleshooting articles and user guides.

Subdomains are also pretty essential if your company operates in different international markets and you want to set up distinct websites geared toward certain countries and regions. If your German customers are forced to navigate the same website as your customers in North America (in English, mind you), that’s not going to lead to a great brand experience. A subdomain saves you the trouble of having to buy a new domain for each country.

In a similar fashion, subdomains are useful when you actually want to create a different brand experience for users. For instance, when companies have different products and services for both consumer and business audiences. If you want to create distinct e-commerce sites for both B2C and B2B audiences (Amazon, for example, has a separate Amazon Business site for B2B users. Although it’s not a subdomain.), a subdomain could be a good option.

Comcast Business is a great example of how subdomains support a better brand experience. Those services are specifically geared toward a B2B audience and are fairly extensive, so it wouldn’t make sense to bury that information somewhere in Comcast’s primary, consumer-focused site. The https://business.comcast.com/ subdomain allows Comcast to create two separate brand experiences for two very different target audiences while using the same domain name.

Companies may also use subdomains to set up mobile-focused variations of their main websites. Obviously, navigating a web page is very different on a smartphone or tablet than a desktop PC. Adaptive development processes dictate that developers account for different screen sizes and form factors when designing mobile websites. Creating a separate subdomain allows businesses to provide an intuitive user interface for both desktop and mobile site visitors.

Should You Use Subdomains?

You’re probably hoping for a pretty cut-and-dry answer here, but there isn’t one.

Like just about everything else related to SEO and Google ranking factors, you have to take things on a case-by-case basis. There’s no set rule dictating the use of subdomains because, for some companies, their benefits will outweigh any potential SEO fallout (either real or perceived).

In other instances, subdomains won’t provide any real tangible value over subdirectories that extend from your main domain, so there’s no sense in making significant changes to the site structure. At the end of the day, it depends on what makes the most sense for your particular website.

John Doherty, founder of the professional SEO and digital marketing provider network Credo, explains that to get the most SEO value out of your subdomain, you need to treat them as if they’re full-fledged websites.

“Subdomains can absolutely be made to work well for SEO, but it requires a lot of extra effort over putting the content (such as a blog) in a subfolder,” Doherty says. “We have seen many times that when content (such as a blog) is moved from a subdomain to a subfolder that often a dramatic increase in organic traffic and rankings occurs. This is harder to do, and maybe even impossible, depending on the purpose of the subdomain, but is something to be considered when launching a subdomain in the first place.”

It’s also important to consider the configuration of your SEO tools and understand how search engines interpret your multiple domains. Tools like Google Analytics and Google Search Console may not default to tracking and reporting metrics across a root domain and subdomain — they’re not recognized as the same, after all.

Instead, cross-domain analytics tracking should be set up to ensure you’re capturing the entire scope of your domains.

In other cases, tools like Semrush, Moz or Ahrefs generally, by default, lump all subdomain and root domain metrics together for ease of use. This level of granularity then informs important benchmarks like Domain Authority.

If you have any questions about your own navigation hierarchy and how to set up your website for optimal ranking opportunities, drop our consulting team a line. We can help identify the best approach to meet your goals and optimize your website.

Editor’s Note: Updated January 2022.