Public Suffix List

The Public Suffix List is a catalog of certain Internet domain name suffixes. A "public suffix" is also known by the older term effective top-level domain (eTLD). The Mozilla Foundation maintains suffix list for the security and privacy policies of its Firefox web browser, though it is available for other uses under the Mozilla Public License (MPL).

According to Mozilla,

A "public suffix" is one under which Internet users can directly register names. Some examples of public suffixes are ".com", ".co.uk" and "pvt.k12.wy.us".

While ".com", ".uk" and ".us" are top-level domains (TLDs), Internet users cannot always register the next level of domain, such as "co.uk" or "wy.us", since these may be controlled by domain registrars. By contrast, users can register second level domains for ".com", such as "example.com", since registrars control only the top level of ".com". The Public Suffix List is intended to enumerate all domain suffixes controlled by registrars.

Some uses for the list are:

• Avoiding "supercookies", HTTP cookies set for high-level domain name suffixes. In other words, a page at foo.example.co.uk might normally have access to cookies at bar.example.co.uk, but example.co.uk should be walled off from cookies at example2.co.uk, since the latter two domains could be registered by different owners.

• Highlighting the most important part of a domain name in the user interface.

• Improving the sorting of browser history entries by site.