In www.example.co.uk, uk is the TLD, co.uk is the public suffix, example.co.uk is the registrable domain, and www is a subdomain. A TLD is always the final DNS label; a public suffix may contain one label or several and marks a boundary beneath which independent users can register names.
Four terms that describe different layers
A domain name is read from right to left in the DNS hierarchy. The root sits above the final visible label. Immediately below the root is the top-level domain, such as .com or .uk. A registry can then organize registrations directly under that TLD or create categories below it.
A public suffix is a suffix under which different parties can register names, now or historically. The Public Suffix List gives examples including com, co.uk and the longer pvt.k12.ma.us. The suffix is an administrative boundary, not a synonym for “the last part after a dot.”
Domain anatomy by example
| Hostname | TLD | Public suffix | Registrable domain | Subdomain |
|---|---|---|---|---|
| shop.example.com | com | com | example.com | shop |
| www.example.co.uk | uk | co.uk | example.co.uk | www |
| docs.project.io | io | io | project.io | docs |
| a.school.pvt.k12.ma.us | us | pvt.k12.ma.us | school.pvt.k12.ma.us | a |
Examples show parsing structure, not ownership or live-site claims.
Why browsers and applications need a list
There is no reliable rule such as “the registrable domain is always the last two labels.” That happens to work for example.com and fails for example.co.uk. Counting labels also fails because registry structures differ across countries and hosted platforms.
The Public Suffix List exists because software needs the real boundary. Browsers use it to prevent a site from setting a cookie on a shared suffix such as co.uk, which would otherwise expose that cookie to unrelated sites. Certificate authorities and infrastructure providers use public-suffix information for policy, rate-limit and account-boundary decisions. The project's technical explanation describes the original supercookie problem and current uses.
Registrable domain is often called eTLD+1
Technical documentation sometimes calls the public suffix an “effective TLD” (eTLD) and the registrable domain “eTLD+1.” The wording is convenient but can confuse people into thinking co.uk is literally a top-level domain. It is not: .uk is the TLD; “effective” means the suffix behaves as a registration boundary for that calculation.
IANA root zone vs Public Suffix List
| Question | IANA Root Zone Database | Public Suffix List |
|---|---|---|
| What does it list? | Delegated top-level domains and their managers | Known registration or administrative suffix boundaries |
| Can entries contain dots? | No visible TLD label contains another dot | Yes; multi-label rules are common |
| Authority | IANA root-zone management | Community resource initiated by Mozilla |
| Main use | DNS delegation and TLD reference | Cookies, domain parsing, policy and site boundaries |
| Private platform entries? | No | Yes, in a separately marked private section |
Implementation guidance for developers
Do not split a hostname on dots and take the last two elements. Use a maintained public-suffix library for the language or platform, preserve Internationalized Domain Name normalization, and update the list on the cadence recommended by the provider. The PSL publishes a current UTF-8 list and says downstream downloaders should fetch it no more than once per day; applications that ship releases can bundle a version and update with the software.
- Parse a URL first; do not treat an arbitrary string as a hostname.
- Lowercase and normalize the DNS name according to the library's contract.
- Keep the distinction between public suffix, registrable domain and full hostname.
- Decide whether the application's policy should include the PSL private section.
- Handle unknown or newly delegated suffixes safely rather than guessing ownership.
For validating whether the final label is actually delegated, use the valid TLD list or IANA data. For registration boundaries, use the PSL. The two checks answer different questions.
Does a public suffix change SEO?
Public-suffix status is not a ranking boost. It can affect how tools group sites, how cookies are scoped and whether a platform treats tenants as separate security boundaries. Search engines make their own indexing and site-understanding decisions. Use one canonical host, link consistently and avoid creating duplicate copies across subdomains; do not use eTLD+1 as an SEO rule by itself.