IP.THC.ORG is a free service providing access to the world's largest Internet domain database, with 5.90 billion+ indexed domains. It offers Reverse DNS (rDNS) lookup, Subdomain finder, and CNAME lookup. No registration or API key is required.

Is IP.THC.ORG free? Do I need an account or API key?

Yes, it is completely free and will always remain free. No registration, account, or API key is required to use the web interface or the API.

How do I perform a reverse DNS lookup by IP address?

Enter an IP address or CIDR range (e.g. 1.1.1.1 or 8.8.8.0/24) in the search box and select Reverse DNS, or run: curl https://ip.thc.org/1.1.1.1. The response lists every domain known to resolve to that IP, with ASN, organization, and location.

How do I find all subdomains of a domain?

Select Subdomains and enter the apex domain (e.g. thc.org), or run: curl https://ip.thc.org/sb/thc.org. The response lists known subdomains drawn from the indexed database.

How do I find all domains pointing to a CNAME target?

Select CNAMEs and enter the target hostname (e.g. github.io), or run: curl https://ip.thc.org/cn/github.io. This reverse-CNAME lookup returns every domain whose CNAME record points to that target — useful for mapping SaaS tenants and detecting subdomain-takeover risk.

How large is the IP.THC.ORG database?

The database currently indexes 5.90 billion+ domains, making it the world's largest free Internet domain database.

Does IP.THC.ORG have an API?

Yes. There are plain-text endpoints (e.g. curl https://ip.thc.org/1.1.1.1), a JSON REST API (POST https://ip.thc.org/api/v1/lookup), and CSV bulk downloads of up to 50,000 records per request. No API key is required. Full documentation is at https://ip.thc.org/llms.txt.

Can I download domain data in bulk?

Yes. Per-query CSV bulk downloads of up to 50,000 records per request are available for each lookup type: reverse DNS via https://ip.thc.org/api/v1/download?ip_address=8.8.8.8, subdomains via https://ip.thc.org/api/v1/subdomains/download?domain=thc.org, and CNAMEs via https://ip.thc.org/api/v1/cnames/download?target_domain=google.com. For offline analysis of the full dataset, snapshot downloads in CSV and Parquet format are also offered — see https://ip.thc.org/docs/bulk-data-access. Bulk download requests consume 3 rate-limit tokens versus 1 for a standard lookup.

IP.THC.ORG — World’s Largest Internet Domain Database

IP.THC.ORG is a free service that lets you search 5.90 billion+ indexed domains — the world’s largest Internet domain database. It provides three reverse lookups in one place: Reverse DNS (rDNS) lookup, Subdomain finder, and CNAME lookup. No registration, account, or API key is required, and it will always remain free.

Reverse DNS lookup (find domains by IP address)

Enter an IP address or CIDR range (such as 1.1.1.1 or 8.8.8.0/24) to list every domain known to resolve to it, together with ASN, organization, city, and country.

curl https://ip.thc.org/1.1.1.1

Subdomain finder (find all subdomains of a domain)

Enter an apex domain (such as thc.org) to enumerate its known subdomains from the indexed database — useful for reconnaissance, attack-surface mapping, and asset discovery.

curl https://ip.thc.org/sb/thc.org

CNAME lookup (find domains pointing to a target)

Enter a target hostname (such as github.io) to find every domain whose CNAME record points to it. This reverse-CNAME lookup helps map SaaS-platform tenants, spot subdomain-takeover risk, and perform competitive intelligence.

curl https://ip.thc.org/cn/github.io

Free API — no key required

Every lookup is available as a plain-text endpoint, a JSON REST API, and a CSV bulk download (up to 50,000 records per request). See the full documentation at https://ip.thc.org/llms.txt.

curl -X POST https://ip.thc.org/api/v1/lookup \
  -H "Content-Type: application/json" \
  -d '{"ip_address":"8.8.8.0/24","limit":10}'

Bulk data download (CSV & Parquet)

Export results as CSV — up to 50,000 records per request — for any of the three lookup types. For offline analysis of the full dataset, snapshot downloads in CSV and Parquet format are available; see bulk data access.

Reverse DNS CSV: https://ip.thc.org/api/v1/download?ip_address=8.8.8.8&limit=500
Subdomains CSV: https://ip.thc.org/api/v1/subdomains/download?domain=thc.org&limit=500
CNAMEs CSV: https://ip.thc.org/api/v1/cnames/download?target_domain=google.com&limit=500

Frequently asked questions

Is IP.THC.ORG free?

Yes. It is completely free with no registration or API key, and will always remain free.

How large is the database?

It indexes 5.90 billion+ domains, the largest free Internet domain database available.

What are the rate limits?

Requests are limited per IP to a 250-token burst with a 0.5 token/second refill. Bulk downloads cost 3 tokens versus 1 for standard lookups.

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