What Is a Subdomain?
Learn how subdomains sit under a domain and get their own DNS answers.
A subdomain is a name beneath another domain in the DNS hierarchy. In blog.example.com, blog is a label under example.com. The operator controlling example.com can usually create that name for a service without registering blog.example.com as a separate domain.
A name for a separate service
Teams often use www for a website, mail for mail infrastructure, or shop for a storefront. Each hostname can have different DNS records and lead to a different server. The existence of example.com does not automatically make every possible subdomain work. A record, wildcard rule, or delegated child zone must supply an answer.
A subdomain can also have deeper labels, such as api.dev.example.com. Read the hierarchy from right to left to understand which name is beneath which.
Records and delegation
You can put an A or AAAA record on a subdomain, point an alias with CNAME where appropriate, or delegate a child zone with NS records. Delegation lets another set of nameservers manage part of the namespace. The exact DNS configuration depends on the service, not on the word chosen for the label.
A CNAME target still has to resolve. A working root domain gives no guarantee that a subdomain has a valid record or that its application is running.
Check the exact hostname
When diagnosing a problem, enter the full hostname at /whois and inspect the relevant record type. Compare blog.example.com and example.com separately; treating them as interchangeable hides configuration mistakes. If you control the DNS zone, check whether a wildcard or explicit entry is responsible for the answer.
An NXDOMAIN response for one subdomain does not mean its parent registration is available. The domain guide explains the boundary between names you register and names you create under them.
Sources
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