What is an eTLD + 1?

In short, an eTLD + 1 is the part of the web address that tells you who owns a website. What does that mean and how exactly can you find it?

1. what is a domain?

The domain is the part of a website address after the protocol (https://) and before the next slash. In a URL like https://jfhr.me/what-is-an-etld-+-1 , the domain is jfhr.me .

2. what is a TLD?

A domain consists of one or more parts separated by dots. The TLD is the last part. In the domain jfhr.me , the me is the TLD.

3. what is an eTLD?

The eTLD or effective TLD consists of the last n parts of a domain that are shared between multiple sites from different owners. In www.google.com , the com is the TLD and also the eTLD - because there are many different pages with different owners ending in com , but all pages ending in google.com belong to google. In www.google.co.uk , the TLD is uk , but the eTLD is co.uk , because there are many different pages with different owners ending in co.uk .

4. what is an eTLD + 1?

The eTLD + 1 is the eTLD plus one more domain part. By definition, the eTLD + 1 and all pages below it belong to the same person or organization. For example, google.com is a eTLD + 1, because everything below it (like www.google.com or drive.google.com ) belong to google.

Diagram showing the domain www.google.co.uk, the part uk is marked as TLD, the part co.uk is marked as eTLD, and the part google.co.uk is marked as eTLD + 1

Schematic diagram of TLD, eTLD, and eTLD + 1.

5. why does it matter?

There is a list called the public suffix list which aims to list all eTLDs on the internet. Browser use it in different ways, but maybe the most important is to restrict cookies. You've probably heard of cookies, they are small pieces of information that websites can store in your browser. Every cookie is associated with a domain, and it can be accessed by that domain and all domains below it. When you visit www.google.co.uk , it can set a cookie for google.co.uk , and when you later visit drive.google.co.uk it can still read that cookie.

But importantly, www.google.co.uk can't set a cookie for just co.uk ! Because that is an eTLD, so it doesn't have a single owner. If you set a cookie there, almost all British websites would be able to read it, which would be terrible for your security and privacy.

You can find the public suffix list here: https://publicsuffix.org/ and under "Learn more" there's also more background.