About

Cybersecurity

Subdomain Checker

Discover and validate subdomains for a domain with a fast reconnaissance-style check.

Features

  • Check common subdomains
  • Review discovered hostnames
  • Spot potential exposures

How to use it

  1. Enter a base domain.
  2. Run the checker.
  3. Review the discovered subdomains and their status.

Example and use cases

Find staging and app-related subdomains that may need additional monitoring or security review.

Useful for reconnaissance, domain inventory, and reviewing exposed service names.

DNS & Security Tool

Subdomain Checker

Find publicly known subdomains associated with a domain using Certificate Transparency data.

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Find Subdomains

Enter a domain to discover publicly known subdomains.

Example:google.com

Discovered Subdomains

Results will appear here

No results yet

Enter a domain and check for publicly known subdomains.

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What is a subdomain?

A subdomain is a domain that exists under a parent domain. For example, inapi.example.com, api is the subdomain andexample.comis the parent domain.

Websites commonly use subdomains for APIs, applications, documentation, dashboards, development environments, staging environments, and other services.

Common subdomain names

Common hostname patterns used by websites and web applications.

www

Main website

api

API services

app

Web application

admin

Administration panel

mail

Mail services

blog

Blog or articles

docs

Documentation

dev

Development environment

staging

Staging environment

test

Testing environment

cdn

Content delivery

status

Service status page

How subdomain checking works

1

Enter a domain

Enter the root domain you want to check, such as example.com.

2

Check certificate data

The tool checks publicly available Certificate Transparency data.

3

Review results

Unique hostnames associated with the domain are extracted and displayed.

Common uses

DNS troubleshooting

Review publicly known hostnames when investigating domain and DNS configurations.

Asset discovery

Create an inventory of publicly observed domain assets for authorized environments.

Security testing

Support authorized reconnaissance and external attack-surface reviews.

Administration

Review publicly visible domain infrastructure and hostname patterns.

Subdomain discovery limitations

This tool uses publicly available Certificate Transparency data. It cannot guarantee that every subdomain belonging to a domain will be found.

Certificate records can also contain historical hostnames that are no longer actively used.

Use this tool only for domains and infrastructure you own or are authorized to assess.

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FAQ

Subdomain Checker FAQ

What is a subdomain checker?

A subdomain checker helps identify publicly known subdomains associated with a parent domain.

What is an example of a subdomain?

In api.example.com, api is the subdomain and example.com is the parent domain.

Can this tool find every subdomain?

No. This tool uses public Certificate Transparency data, so some subdomains may not appear in the results.

What is subdomain enumeration?

Subdomain enumeration is the process of identifying subdomains associated with a domain.

What is Certificate Transparency?

Certificate Transparency provides publicly auditable records of TLS certificates issued by certificate authorities.

Does finding a subdomain mean it is vulnerable?

No. Discovering a hostname does not establish that the associated service is vulnerable.

Can this tool find private subdomains?

Private or internal hostnames may not appear in public Certificate Transparency records.

Can results include inactive subdomains?

Yes. Certificate records can contain hostnames that are no longer actively used.

Is subdomain checking legal?

Only perform security testing or intrusive scanning against systems you own or are authorized to assess.

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Frequently asked questions

Is this a full subdomain enumeration service?

It provides a quick browser-based discovery check and should be used responsibly with permission on the target domain.

Related developer tools

DNS LookupSSL Certificate CheckerWHOIS Lookup