Free Tools/Subdomain Finder
Technical SEO

Subdomain Finder

Discover subdomains associated with any domain, sourced from public SSL certificate records. Find hidden subdomains, competitor sub-sites, and forgotten staging environments.

Why use our Subdomain Finder

Discover subdomains instantly

Enter a domain and see every subdomain that has ever had an SSL certificate issued, no manual DNS lookups required.

Free subdomain finder, no signup

Discover subdomains for any domain online with no account required, sourced from public certificate transparency logs.

Surfaces forgotten staging environments

Finds old test subdomains and microsites that can expose duplicate content or security risks if left unmanaged.

Competitive subdomain intelligence

See a competitor's subdomain structure, revealing separate product lines or regional sites you didn't know existed.

Results come from public SSL certificate transparency logs via crt.sh, covering any subdomain that has ever had a certificate issued.

Real Subdomain Finder example: before vs. after

A company running a subdomain audit for the first time in years.

Before · Unknown subdomains
  • old-staging.example.com → still live, indexed
  • test2019.example.com → forgotten test site

Two forgotten subdomains were publicly accessible and indexed, creating duplicate content and a security exposure.

After · Cleaned up
  • old-staging.example.com → removed
  • test2019.example.com → redirected

Removing or redirecting the forgotten subdomains closed the duplicate content and security risk.

Frequently Asked Questions

What is a subdomain?
A subdomain is a prefix added to a domain name, like blog.example.com or shop.example.com. Each subdomain can function as a separate section of a website, sometimes with entirely different content or hosting.
Where does this tool get its subdomain data from?
It queries public SSL certificate transparency logs (via crt.sh), a public record every certificate authority is required to log. Any subdomain that has ever had an HTTPS certificate issued will show up here.
Are subdomains bad for SEO?
Not inherently. Subdirectories (example.com/blog) are generally better for SEO because they consolidate authority under one domain. Subdomains work fine for genuinely separate products, but Google can treat them as distinct sites.
How do I find all of my own site's subdomains?
Run this tool against your own domain, then cross-check the results against your DNS records in your domain registrar and your Google Search Console property list.
What should I do with unused subdomains?
Remove or redirect them. Unused subdomains waste crawl budget, can create duplicate content issues, and may expose forgotten staging environments to search engines.

Once you know your subdomains, check each one with our Sitemap Checker to confirm search engines can crawl it correctly.

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Free Subdomain Finder: Discover All Subdomains for Any Domain | SEOMentor