A subdomain is an extra name you put in front of a domain you already own, such as book.example.com or help.example.com. It lets one domain host several separate sites or services, and you can create as many as you like without registering anything new.
How a subdomain is created
A subdomain is just another entry in your domain's DNS zone. You open the DNS settings wherever your nameservers live, add a record with the name book, and say where it should go. Usually that is either an A record holding an IP address or a CNAME pointing at another hostname, like yourstudio.bookingtool.com. Once that record is live, book.example.com resolves on its own, independently of the main site.
Strictly, www is itself a subdomain. That is why www.example.com and example.com can show different things if they are not set up to match.
Example: a dance school with three services
A dance school keeps its marketing site on example.com, runs class booking through a scheduling service on book.example.com, and hosts a parent help centre from a support tool on help.example.com. Each service gives the school a hostname to point a CNAME at, and visitors see one consistent brand across three different providers.
Subdomain or subfolder?
The alternative is a path such as example.com/blog. A subfolder is part of the same site, so its content clearly sits with your main pages. Google has said it can handle either structure, but subdomains are often tracked as separate properties in Search Console and analytics, and links earned by one do not obviously help the other. Use a subdomain when a different system genuinely runs that section. Use a subfolder when you just want another section of the same site.
When you need one
Most small businesses only touch subdomains when a third-party tool asks for one: a shop platform, a booking app, a newsletter archive or a customer portal. If your site is a single system, you may never add any.
Common subdomain mistakes
- Forgetting the certificate. Each subdomain needs to be covered by an SSL certificate, either its own or a wildcard, or browsers will show a security warning.
- Leaving dead records behind. A CNAME still pointing at a cancelled service can let someone else claim that service name and serve content on your subdomain. Delete records when you stop using a tool.
- Splitting a blog off for no reason. Putting articles on a subdomain because the theme was easier can scatter your search signals.
- Typing the full name into the record. Many DNS panels add the domain automatically, so entering
book.example.comcreatesbook.example.com.example.com.
Related terms
- Domain name: the registered name every subdomain sits under.
- CNAME: the record most often used to point a subdomain at a service.
- DNS records: where subdomains are defined.
- SSL certificate: needed for each subdomain to load over HTTPS.