Reference
The Public Suffix List
The Public Suffix List (PSL) is a volunteer-maintained list, hosted by Mozilla, of every suffix under which the public can directly register names — all real TLDs, plus deeper suffixes like co.uk or provider suffixes like github.io.
Why it matters
- Cookie boundaries
- Browsers use the PSL to stop one customer of a shared suffix from setting cookies for everyone else on it.
- Certificates & wildcards
- CAs refuse wildcard certificates at or above a public suffix.
- What counts as "a site"
- Rank trackers, ad systems, and security tools group hostnames by "registrable domain", which the PSL defines.
How dom4in uses it
The PSL is one of our cross-checks that a TLD is real and publicly registrable, alongside the IANA root zone.
Browse the list at publicsuffix.org (external).