What is a subdomain and how do you set one up?

A subdomain is a name within a wider domain hierarchy. In blog.example.org, blog sits under example.org. Subdomains can separate a website, mail, an API and other services, but the name alone creates no web page. You still need DNS records and a server configured to accept it.

Subdomain or path?

blog.example.org is a separate hostname, while example.org/blog is a path on the example.org website. The two can display similar content but are configured differently. A subdomain can point to another server, be served by another application or be delegated to a separate team. A path generally stays under the same hostname, although the application behind it can still be independent.

There is no universal rule that subdomains are better for search or security. Consider architecture, access control, language handling and maintenance. For a section related to the main site, plan navigation, internal links and canonical URLs regardless of the chosen structure.

Choose the records

A website commonly uses A or AAAA to provide a server address, or CNAME to alias one name to another. CNAME has rules about coexisting with other records at the same name. If the subdomain receives mail, configure its MX and associated records according to the mail provider’s requirements. Do not copy the parent domain’s records automatically: the subdomain may serve a different purpose.

A wildcard such as *.example.org can answer for some otherwise undefined names, but it cannot replace application routing or every explicit record. A subdomain with its own DNS zone needs delegation through NS. RFC 1034 and RFC 1035 provide the DNS foundations.

Verify the service

Decide what the name represents and which server will accept it. Add the record to the active authoritative zone, then inspect it with the DNS configuration checker. If DNS returns the expected IP but the site does not open, check the virtual host, application routing and TLS certificate for the new name.

For a login page, a separate subdomain alone is no security boundary. Configure authentication, cookies, permissions and TLS in the application. Moving an existing section to a subdomain also requires a plan for old URLs and internal links.

Check the name itself

Lookalike characters and typos can lead to a different domain. Internationalised names have a DNS-compatible ASCII form; the Punycode converter helps inspect it. Do not enter a password on an unexpected subdomain just because its ending resembles a familiar address.