A subdomain – or child domain – is a part of a website’s domain that sits under the main ‘parent domain’. Subdomain can be used for any number of reasons, such as organising certain sections away from the main site, to make navigation easier or more intuitive, or simply to distinguish it as having a separate function from the main site. An example of this would be a site to which the owner wishes to attach an ecommerce store without obscuring the main business of the site. In this example if the site is called ‘mywebsite’ and its top level domain is ‘.com’ then a retail subdomain could be added as ‘shop.mywebsite.com’.
Apart from the ecommerce example described above there are many other reasons why a site owner might use a subdomain. It is very common for web owners and designers to use a subdomain in the beta stage of development. It becomes a place where plugins, updates and any other new experimental features can be tested before being pushed to the live site.
Many companies used to develop their mobile sites using a subdomain as there was no need for redirects, giving the user a faster experience (although tanks to responsive websites this is less common nowadays). Slow response times are one of the biggest deterrents to using mobile. Similarly, subdomains are ideal for locally-focused uses, sub-sections of specific interest and for defined groups of users including subscribers or past customers.
Most reputable hosting services offer the facility to create subdomains through their account dashboards. In effect, it is no more completed than simply adding a new page: it is only its relationship to the main site which is different.
Subdomains shouldn’t be confused with subdirectories which refer to the more common organisation of pages on a domain and follow the main domain name after a forward slash (eg ‘mywebsite.com/shop’).
Subdomains and SEO
The major search engines treat subdomains as separate websites, distinct from the parent site. The crawlers will need to index the subdomain just as if it were a brand new site and so a subdomain will not automatically benefit from SEO techniques such as backlinks which are pointing to the main site. Building rankings at the page level will effectively start from scratch then, which is a good reason to limit the use of subdomains to circumstances in which there is no viable alternative such as a dedicated page or sub-directory.