You've probably seen website URLs that look something like this:
Web pages with URLs that look like this are known as subdomains.
Major websites like Amazon and Wikipedia use them frequently, and even some smaller websites may use them.
Below you'll learn what a subdomain is and why people use them to help improve both the user experience and SEO value.
When you finish building your website, you buy a domain name. Within the domain name, there are many different parts. For example, in www.example.com, ".com" is called a top-level domain. There are many top-level domains including ".org", ".net", and ".gov".
If your domain name is www.example.com, "example" is called a secondary-level domain. Finally, if you have a URL like www.blog.example.com, "blog" is a third-level domain, more commonly known as a subdomain. You don't have to purchase a subdomain, just the domain name. However, just because you can create a subdomain doesn’t mean that you should create one.
Before discussing why you might use a subdomain, it's important to realize that subdomains impact SEO.
Even though the user will see your homepage and subdomain as the same website (with a slightly different URL), search engines will see them as entirely different websites. Therefore, any links and content you've built for the main website will not transfer to the subdomain and therefore, could weaken your SEO strategy. Despite this, there are still some uses for subdomains.
One reason you may want a subdomain is for members only areas. These areas and pages don't need to be a part of your website as they aren't generating links or traffic, and you may also no-index most of them.
Traditionally, people also used subdomains for blogs. However, this is now a dated practice as most SEO professionals agree that it's better to use a subdirectory (www.example.com/blog) rather than a subdomain (www.blog.example.com) for the link juice and traffic reasons mentioned above.
If you work with a major corporate brand with offices/stores across the country, using various subdomains allows you to target local keywords more effectively. If you have multiple locations across the country and each one targets a certain set of keywords, you may choose to have separate subdomains.
For example, Craigslist is listed as state.craigslist.com so that the individual page is hyper-targeted for that particular state, giving it a better chance at ranking.
Larger websites containing vast libraries of information on various topics often use subdomains to help Google understand each section's topic.
One example is Harvard University which contains a lot of authoritative information on various topics. Instead of housing all information on one domain, they create separate subdomains dedicated to each topic.
For example, all health-related information is listed on a health.harvard subdomain. By stripping away all other information that is irrelevant to health, it creates an easier user experience. Google will also rank it higher for health-related terms as it focuses solely on health.
Sometimes web developers will recommend that you create a test version of your website on a subdomain. This allows you to see how it would perform live without the risk of damaging your real website. You can block the test subdomain with a password that prevents both bots and people from accessing it and allows your current website to run without a hitch.
Most websites never have to use a subdomain. If you have a brand new website, make everything a subdirectory rather than a subdomain and only talk to a web developer once you grow above 10,000 visitors per month. Keeping your strategy simple and consistently creating content will help you much more than spending time on SEO hacks like subdomains in the early stages.