What is a subdomain?
A domain name has levels, read from right to left. In blog.example.com, the .com is the top-level domain, example is the name the owner registered, and blog is a subdomain. The owner can create as many subdomains as they like without registering anything new, simply by adding records in the domain's DNS settings. The familiar www at the start of many addresses is itself a subdomain, a convention left over from the early web.
Each subdomain can point to a different server, run different software and be managed by a different team. That is why they are used for things that live apart from the main site: a help center on a support platform, a store on an e-commerce service, a staging copy for testing, a regional version, a web app at app or a login portal at account.
Why it matters
Why a subdomain matters
Subdomains give flexibility, and they carry trade-offs that are easy to miss. Search engines treat each subdomain largely as a site of its own, so content on blog.example.com may not benefit as directly from the main site's authority as it would at example.com/blog. That debate has run for years: Google says it handles both well, while many SEO practitioners have seen content gain from moving into a folder. Subdomains also split analytics and cookies by default, and every one that is created has to be looked after, because a forgotten subdomain pointing at a service no longer in use can be taken over by someone else.
In practice
How a subdomain works in practice
- 01
Creating one is a DNS record: an A or AAAA record pointing the subdomain at a server's address, or a CNAME pointing it at another hostname, such as a platform's service. The platform at the other end is configured to answer for it, a certificate is issued so it runs on HTTPS, and analytics are set to track across the main domain and its subdomains, so visitors moving between them are not counted twice.
- 02
The bigger decision is whether to use one at all. We put content that should rank, such as articles, guides, a glossary or case studies, in folders on the main domain, where it builds the main site's authority. Subdomains are for things that are technically separate or should be kept away from search: the app, the help center, staging, internal tools. And every subdomain goes on a list, so it is removed from DNS when the service behind it is retired.
Watch out
What people get wrong about subdomains
Putting the blog on a subdomain because the platform made it easy, then wondering why it ranks poorly and does little for the main site. Second, leaving staging subdomains open to search engines, so an unfinished copy of the site competes with the real one. Third, forgetting old records. A subdomain still pointing at a deleted cloud service or a canceled platform is an open invitation to subdomain takeover, where someone else claims the service and publishes their content under your name.
