Glossary Guides

What Is a Subdomain

Learn what subdomains are, common examples like www and mail, DNS records per subdomain, and SSL considerations.

By CheckDomainHealth Editorial Team Reviewed by Dionis Ceban Updated Jun 28, 2026 5 min read Beginner

Introduction

Subdomains let you organize services under one brand. The root domain example.com and subdomain www.example.com are separate DNS names that can route to different servers.

Subdomains are not separate domain registrations — they are DNS names inside your existing zone, unless delegated to another DNS provider.

Quick answer

Quick answer

A subdomain is a hostname under your main domain, such as www.example.com, mail.example.com or app.example.com. Each subdomain can have its own DNS records (A, CNAME, MX, TXT) and may point to different services.

What it means

A subdomain adds a label before the registered domain. DNS treats each full name as its own hostname with its own records.

  • www is the most common subdomain
  • mail, app, api, staging are frequent use cases
  • Each subdomain can have A, AAAA, CNAME, MX or TXT records
  • SSL certificates must cover subdomains you use (or use wildcard)
  • Wildcard DNS (*.example.com) can catch undefined subdomains

Where you see this:

  • www for websites
  • mail for mail server hostnames
  • app or api for applications
  • staging or dev for test environments
  • CDN or SaaS verification subdomains
Example 
www.example.com
mail.example.com
app.example.com
blog.example.com

Why this matters

Why this matters

Missing subdomain DNS breaks only that hostname — the root site may work while app.example.com fails. SSL and hosting must be configured per subdomain when needed.

How to check it

  1. Run DNS Lookup on the full subdomain (for example app.example.com).
  2. Check which record type it uses: A, AAAA or CNAME.
  3. Confirm SSL covers the subdomain if it serves HTTPS.
  4. Verify hosting or CDN mapping for that hostname.
  5. Test the URL in a browser or with Website Status Checker.

Check subdomain DNS

Use DNS Lookup to see how a subdomain resolves and which records exist.

Run DNS Lookup →

Common mistakes

Subdomain DNS record missing

High

The subdomain name was created in hosting but no DNS record exists.

Next step: Add A, AAAA or CNAME for the subdomain at authoritative DNS.

SSL does not cover subdomain

High

Certificate is valid for example.com but not www or app.

Next step: Include subdomains in the certificate or use a wildcard cert.

CNAME conflict on subdomain

Medium

CNAME on a name that also needs MX or TXT records.

Next step: Restructure DNS so conflicting records use different hostnames.

Wrong document root or hosting mapping

Medium

DNS resolves but hosting serves the wrong site or 404.

Next step: Configure virtual host or panel addon domain for the subdomain.

Example

Subdomain DNS examples 
app.example.com.   300  IN  A      192.0.2.20
www.example.com.   300  IN  CNAME  example.com.
mail.example.com.  300  IN  A      198.51.100.5

Frequently asked questions

Is www a subdomain?

Yes. www.example.com is a subdomain of example.com.

Do subdomains need separate registration?

No. Subdomains are created with DNS records inside your existing domain zone.

Can subdomains use different hosting?

Yes. Each subdomain can point to a different IP or external service via DNS.

What is a wildcard subdomain?

A record like *.example.com that matches any undefined subdomain name.

Use these free tools to verify your configuration after applying changes.

DNS Lookup Domain Health Checker SSL Checker
Setting Up Subdomains: DNS Records and Best Practices 7 min read What Is an A Record 5 min read What Is a CNAME Record 5 min read Wildcard DNS Records: Use Cases and Configuration 7 min read Understanding DNS: How Domain Names Connect to Websites and Email 8 min read
Browse all Glossary guides →

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