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ChatGPT & The Public Suffix List ​

The Public Suffix List (PSL) is an open-source dataset maintained by Mozilla that defines where domain boundaries exist. ChatGPT uses the PSL to determine domain ownership, and this has direct implications for how authority is attributed in AI search.

What the PSL Does ​

The PSL defines "effective top-level domains" (eTLDs). It tells software where one domain ends and another begins.

Without the PSL:

  • blog.example.com and shop.example.com look like separate sites
  • user123.github.io looks like a subdomain of github.io
  • mysite.co.uk might be parsed as a subdomain of co.uk

With the PSL:

  • blog.example.com and shop.example.com are recognized as the same organization (example.com)
  • user123.github.io is recognized as a separate entity from user456.github.io (because github.io is on the PSL)
  • mysite.co.uk is correctly parsed as a standalone domain

PSL Categories ​

CategoryExampleMeaning
ICANN domains.com, .org, .co.ukCountry and generic TLDs
Private domainsgithub.io, herokuapp.com, blogspot.comPlatform domains where users get subdomains

Technical Implementation in ChatGPT ​

ChatGPT's search system uses the PSL at multiple stages:

Domain Grouping ​

When ChatGPT retrieves search results, it groups results by domain. The PSL determines the grouping boundary:

  • docs.example.com and blog.example.com are grouped as one domain (example.com)
  • user1.github.io and user2.github.io are treated as separate domains (because github.io is on the PSL)

This affects the grouped_webpages source type documented in RRF & AI Citations. Results from the same effective domain get grouped together, potentially boosting aggregate RRF score.

Domain Diversity Filtering ​

ChatGPT limits how many results from the same domain appear in each sub-query's ranked list. The PSL determines what "same domain" means:

  • Multiple pages from example.com (including subdomains) count as one domain
  • Multiple pages from different github.io users count as separate domains
  • Multiple pages from different wordpress.com blogs count as separate domains

Authority Attribution ​

Domain-level authority signals (backlinks, brand recognition, training data presence) are attributed at the effective domain level:

  • Authority earned by blog.example.com benefits shop.example.com
  • Authority earned by user1.github.io does NOT benefit user2.github.io

SEO Implications ​

Platform Attribution ​

Publishing on Platform Subdomains

If you publish content on a platform domain listed in the PSL (GitHub Pages, Blogspot, WordPress.com, Substack, Medium), the authority you build stays with your subdomain -- not with the platform. This means:

  • Your username.github.io site builds authority independently
  • You are not borrowing github.io's domain authority
  • You are not contributing to github.io's domain authority
  • Each platform subdomain is effectively an island

Ownership Boundaries ​

Content published across subdomains of a domain you own rolls up to your root domain:

SetupAuthority Rolls Up?
blog.yourdomain.comYes -- benefits yourdomain.com
docs.yourdomain.comYes -- benefits yourdomain.com
yourname.medium.comNo -- medium.com is on the PSL
yourname.substack.comNo -- substack.com is on the PSL
yourname.github.ioNo -- github.io is on the PSL
yourname.wordpress.comNo -- wordpress.com is on the PSL

Authority Dilution ​

Publishing on multiple platform subdomains dilutes your authority across multiple effective domains instead of concentrating it:

Diluted strategy:

  • Blog on yourname.medium.com
  • Docs on yourname.gitbook.io
  • Landing pages on yourname.webflow.io
  • Projects on yourname.github.io

Each of these is a separate effective domain. Authority does not flow between them.

Concentrated strategy:

  • Blog on blog.yourdomain.com
  • Docs on docs.yourdomain.com
  • Landing pages on yourdomain.com
  • Projects on projects.yourdomain.com

All authority rolls up to yourdomain.com. Domain grouping in ChatGPT's search treats all of these as one entity.

Recommendations ​

  1. Host everything on your own domain. Subdomains of your root domain consolidate authority. Platform subdomains do not.

  2. If you must use platforms, cross-link to your domain. Content on Medium or Substack should link back to your root domain. The platform content will not transfer authority directly, but links from platform content can signal relevance.

  3. Check the PSL before choosing a hosting platform. If the platform's domain is on the PSL, your subdomain is treated as a separate domain. Verify at publicsuffix.org.

  4. Consolidate existing platform content. If you have valuable content scattered across multiple platform subdomains, consider migrating it to your own domain with proper redirects and canonical tags.

  5. Understand the trade-off. Platform subdomains offer ease of use and sometimes their own distribution (Medium's recommendation engine, Substack's newsletter network). The authority trade-off is real but not always the deciding factor. Choose based on your strategy.

The Core Principle

The PSL determines domain boundaries in ChatGPT's search. Everything on your root domain is one entity. Everything on a PSL-listed platform domain is its own island. Structure accordingly.