Choosing between subdomains and subdirectories creates a lot of disagreement in the SEO community.
Let’s start with the basics – here's how subdomains and subdirectories present differently as URLs:
What's the difference between a subdomain and a subdirectory?
The subdomain is a subsidiary of the parent domain. On the other hand, the subdirectory is more of a subfolder that indicates that the page is a part of the main domain.
The subdomain technically resides separately from the main domain – it's its own division of the domain. A subdirectory is a simple website page linked to your main domain.
In terms of the website address, you can tell if the page you’re on is a subdomain or a subdirectory based on whether the page sits before the root domain (info.startup.com) or whether it sits after the root domain (startup.com/info).
You can check what subdomains/subdirectories your site is currently using with the Semrush Site Audit Tool.
So which is better – subdomains, or subdirectories?
Many are justifiably confused about which website structure to use because they’ve heard (correctly) that it can impact your site’s organic search performance.
Google treats subdomains as separate from your main domain for the purposes of search ranking. This can either be a good or a bad thing, depending on whether the section of the site should be seen as a separate entity.
For example, you may want to keep the internet traffic associated with a blog section of your site separate from the main domain. On the other hand, making all sections of the site separate subdomains wouldn't make sense in a lot of cases.
For example, users frequently navigate between the ‘About Us’ page and the ‘Contact Us’ page on a website. To have them be separate subdomains would disrupt the natural flow of traffic on your website.
To be clear, Google won’t penalize use of subdomains or subdirectories, regardless of your setup. However, there can be advantages to each.
Changing subdomains to subdirectories when they're relevant to each other can improve traffic to your domain. This is because the subdirectory is receiving an SEO boost from the search traffic already associated with the main domain on Google.
Are subdirectories better than subdomains then?
Subdirectories can present their own set of challenges. For starters, you can't set up an international site as a subdirectory (i.e. startup.com/uk). It's also worth noting that certain forms may not be able to occupy the same server as the root domain if they're written in different coding languages.
However, some CMS platforms may require the use of a subdomain. For example, if you host your blog on Wordpress, you can either use their default subdomain (yourblog.wordpress.com), or link a custom domain that you've purchased.
To summarize, in most cases the ideal scenario is to structure your site using subdirectories, unless the page of the site isn't compatible with subdirectories. Subdomains are also an excellent way to structure blog and other ancillary content collections that may not fit with the overall content of the primary domain.