Subdomains are older than the World Wide Web. RFC 1034, adopted in November 1987, defined subdomains this way: “A domain is a subdomain of another domain if it is contained within that domain.” Seems simple enough. And yet generations of Internet pontification have disagreed on whether “www.[whatever]” is a subdomain or something else.

To a search engine there is a URL – a Uniform Resource Locator as defined in RFC 1738 (adopted in December 1994): “A URL contains the name of the scheme being used () followed by a colon and then a string (the ) whose interpretation depends on the scheme.”

We’ve learned to think of URLs as addresses. You type a URL into the Web browser address bar. You bookmark a Web address. You might trace a path to a Website by its IP address. If you put a in front of a domain name or an IP address you have a URL. So it’s not quite what the colloquial uses make it out to be.

Whether you publish content at www.domain.tld, sub.domain.tld, or domain.tld/subfolder does not matter to a search engine. Each of those strings points to only 1 thing: a document. That’s a single document, not a Website, nor even a host.

Websites and hosts are abstract concepts. They don’t exist in the sense you and I visualize. They exist only according to the rules we agree to apply to our abstract concepts. That’s important to any discussion of whether subdomains help, hurt, or have no effect on search engine optimization. You can define a subdomain to be beneficial or not beneficial to search engine optimization. There’s no inherent up or down, yea or nay, good or bad.

The idea that search engines treat subdomains differently from subfolders is an SEO myth – an assumption treated as a fact because it helps (some) people understand how Websites work (in a certain way).

The thing is, there is no way to truly distinguish a subdomain from its parent domain. You can set up a Website (domain.tld) in a folder on a Web server (public_html) such that all its subdomains (a.domain.tld, b.domain.tld, c.domain.tld) resolve to the same server folder (public_html). In fact, I do this all the time with WordPress multisite subdomains. They don’t need their own sets of files and server resources.

To serve content to any remote agent, all a Web server needs is a map between a URL and something on the server. And if you’re clever, you can use a real-time resolver to intercept URLs before the Web server tries to find it in its list of URLs. This is how wildcard subdomains are managed – via some application running on top of the Web server application. WordPress is the most common choice for creating wildcard subdomains.

Search Engines Group URLs Together

Although one might reasonably assume I mean search engines aggregate URLs together to identify “sites”, that’s not what I’m saying. Suppose you have 3 URLs like this:

www.alpha.tld
www.beta.tld
www.gamma.tld

They look like 3 separate, distinct domains. They must be 3 hosts. You can even go to the trouble of hosting them on separate servers.

But if you publish the exact same content on all three domains/hosts, the search engines may group them together through inferred canonicalization. Or you could link alpha and beta canonically to gamma, so the search engines treat them all as if they are the same document.

The system just isn’t fair to people who want subdomains and subfolders to be completely different things, where one has an advantage over the other. A subfolder may be defined in any part of a domain name. These two subfolders are equivalent:

www.alpha.tld/subfolder
sub.alpha.tld/subfolder

Neither URL has an advantage over the other. The search algorithms don’t say, “Aha! one has a ‘www.’ in front of it so it must be more important than the other.”

And what about URLs that lack any subdomain-like name fragment?

Is alpha.tld more important to a search algorithm than www.alpha.tld?

The rules of host name hierarchies as proposed by careless Web marketing bloggery don’t make any sense. In terms of optimizing for search, it really doesn’t matter (to a search engine) because it doesn’t need to matter if your content is served from en.wikipedia.org or www.wikipedia.org.

What matters to the algorithms is how you prioritize your URLs, how you link them together, and whether you associate any specific rules with them such as “rel=’canonical’”, “nofollow”, “noindex”, “disallow”, etc.

So, now that we’ve established it’s only what you make of your URLs that matters, let’s talk about the pros and cons of using subdomains for your URLs. Obviously we’re organizing content into some kind of logical hierarchy. That’s really the best way to think about the formats of URLs. How you’re organizing the content you consider to be “part of your Website”.

Pro: You Can Use the Same URLs Multiple Times

If you have 3 hosts/subdomains on your Website, and you really like the page name privacy-policy, you can use that on all 3 subdomains. Each page name completes a unique URL.

This example is relevant to any multi-site network, such as a Web hosting service that gives clients a subdomain for their sites. Everyone can have a privacy policy if they wish.

Other types of businesses might have public and private subdomains, each with their own “about” page, or “terms and conditions”.

Or you could do like we did with SEO Theory and move your old blog posts to a subdomain archive. That allows you to re-use particularly good URLs if you don’t want to redirect them.

Pro: You Can Host Subdomains on Different Servers

This often proves to be a necessity for large sites that integrate 3rd-party interfaces like shopping carts or Web forums into their content.

Many small companies host their shopping systems on third-party platforms and integrate their URLs (and often their designs) into a sitewide scheme. The site spans 2 or more hosts or subdomains but it’s a single site.

That was how I managed a large body of content on Xenite.Org in the 1990s, when Web hosting was more expensive and less robust than it is today.

Many companies put paywalled content on a subdomain, or use accounts. All of these things can be served by other systems. The SEO benefit is derived from distributing your service load across multiple machines. You don’t have to worry so much about the site running slow or a single error taking down the entire site.

Pro: Subdomains Can Have their Own Content Management Systems

Subfolders can have their own unique CMS installations, but if you’re using the fast install tools Web hosting companies provide you must live with their assumptions.

Also, many CMS’s intercept all URL requests below their installation folders and I’ve occasionally run into conflicts.

I see people ask for help with these hybrid CMS mixes several times a year. It happens more often that most of us realize. Usually a separate subdomain is needed if it requires dedicated disk space and other server resources. Again, this could be a shopping cart or a private customer area that is managed by different software from whatever is publishing the main Web content.

The SEO benefit is that you can construct unique sitemaps, site navigation, and default HEAD section content per each subdomain’s requirements without having to worry about accidentally reusing something on the wrong URL.

You can choose mnemonic subdomain names, like shop.domain.tld, user.subdomain.tld, etc.

Pro: Subdomains can stand on their own

In many situations, subdomains earn more brand value than their root domains. There is nothing wrong with that. The most successful example I can think of is Wikipedia’a English-language subdomain.

20 years ago the root domain of Xenite.Org attracted the most traffic from search and Website referrals. But that’s changed over the past decade. Most of the site’s visitors are now looking for the Middle-earth blog.

Embrace success wherever it finds you and leverage it to benefit your entire site. Don’t create artificial success by forcing people to squeeze into 1 specific page or host name.

Pro: Subdomains Save You Money

If you have a root domain name that works well with words in front of it, you can create brandable subdomains for free. You don’t need 1 million subdomains for SEO. You do need good, memorable, easy-to-type host names for your root and subdomains.

Depending on how you implement the subdomains, you may not have to pay for extra hosting resources. That’s a plus. especially for people who aren’t sure about how much they should be spending for hosting.

Pro: Subdomains Can Have Their Own SSL Certificates

This is a minor advantage. But if the certificate on the root domain expires, the certificates on the subdomains may still remain in effect. It depends on whether you acquire the certificates at the same time.

If one host is hacked and the certificate is compromised, other hosts may not be. Having separate certificates doesn’t guarantee anything except that hackers will have to do more work to compromise your entire site.

Modern Web hosting dashboards like CPanel, Plesk, Web/Virtualmin, and others can install free SSL certificates automatically. Many CPanel users get Comodo certificates (you pay may pay extra for the feature), but you can also set up free AutoSSL options from Cloudflare, Let’s Encrypt, ZeroSSL, and others.

Con: Combining Analytics Is A Chore

You can rollup multiple accounts into one account. You may have to pay extra for the privilege, depending on your analytics package. Free analytics tools usually don’t allow this.

But how you do you distinguish between these 3 URLs in your analytics?

That’s how the root pages for a.domain.tld, b.domain.tld, and c.domain.tld look in analytics reports.

If you put all your PPC landing pages on a subdomain, do you really need to combine their traffic data with the unpaid search referral data for the rest of the site?

If you’re splitting your content across multiple subdomains, it’s better to keep the analytics reporting as independent as possible.

Con: Less Informed People Second-guess Your Decision

Whether you’re a site owner who wanted to leverage the advantages of using a subdomain or an SEO provider who found solid justification for building up subdomain success, sooner or later someone who “knows better” comes along and tries to undo all the good work.

The greatest peril is for small business owners who change SEO providers or hire new SEO employees. If the new specialist looks at your subdomains and says you should move them to subfolders, give that person a chance to understand why things are as they are.

If they insist that subfolders outperform subdomains, tell them politely but firmly, “You’re welcome to your beliefs but as my SEO provider you must support my business decision.” A good SEO provider will learn about your site’s history and structure and understand why things are as they are before presenting you with “a better plan”.

Con: Subdomains Require More Work

Setting up a Website or host is relatively simple. Maintaining them may be another story. Although you can often share databases and other resources between subdomains, each new resource requires a little bit of maintenance work.

Do you need separate SSL certificates, databases, Content Management Systems, email services, etc.?

It’s more efficient to keep as many things under one roof as possible.

Balance: Search Engines Treat Subdomains and Subfolders the Same Way

Many people are quick to point out that “subdomains are treated as if they are separate sites.” Not exactly.

The truth is that a search engine may list any subfolder on any host (root or subdomain) as if it’s an independent Website: showcasing it with in-SERP Site Search, Sitelinks, and other special indicators suggesting it’s a standalone Website.

You can even verify subfolders independently in search engine Webmaster dashboards, or assign them unique analytics IDs in many different Analytics tools.

Neither subdomains nor subfolders have any special advantages or disadvantages in search indexing algorithms. They are just URLs to the crawlers and search database systems.

Balance: Subdomains And Subfolders Provide the Same User Experience

Web browsers don’t care about where the content is hosted. They do need to resolve each host (subdomain) name, but that only takes a microscopic amount of time. Unless you’re generating subdomain names on the fly (and linking to them), you’re not going to use up much client or server time by serving content from multiple subdomains.

Your visitors may not even realize they are seeing content from multiple hosts or servers. Everything could be using the same layout, theme, and other resources. So to the visitor – as to the browser and search engine – it’s all the same.

If you choose to associate brand value with a URL, it’s best to choose the URL structure that is easiest to remember.

Personal preference matters far more than anything else when it comes to deciding whether to use subdomains or subfolders. As long as you don’t weaken the interconnectivity between the URLs on your site with “rel=’nofollow’” attributes, “noindex” or “disallow” directives, or improper canonicalization the search engines will happily crawl your site and treat all the URLs the same regardless of where they are found.

The PageRank-like value that flows into your URLs is weighted the same way for subdomains as for subfolders. The algorithms don’t differentiate link value by destination host.

What I usually advise people to do is choose the URL structure that is easiest to brand. But they should also think about what they need to do to implement the full functionality they want.

You may decide to avoid subdomains to keep things simple. Or you may decide to use subdomains because that simplifies things. Either way, you don’t need to worry about hurting your SEO.

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