A top level domain (TLD) is the part of the domain name following the final dot: so for example, the top-level domain in crookedtimber.org is org .

Suppose only domains directly under top-level domains were registrable by single organizations. Then you would know that the following domains all belonged to the same organization:

xyz.org
abc.xyz.org
def.xyz.org

However, this does not work as a general rule, because many registrars allow organizations to register domains at levels below the top level. This means that, for example, sussex.ac.uk and aber.ac.uk are owned by different organizations.

Because this is a matter of the registrar's policies, it's impossible to tell algorithmically whether a given domain name suffix (like ac.uk ) is publicly registrable or not. The Public Suffix List is a list of all suffixes under which organizations can directly register names: that is, it is a list of eTLDs.

The related concept eTLD+1 means an eTLD plus the next part of the domain name. Because eTLDs are registrable, all domains with the same eTLD+1 are owned by the same organization.

For example, all the following are eTLD+1 domains:

• crookedtimber.org
• theguardian.com
• sussex.ac.uk
• aber.ac.uk

This means that all domains under each of these domains belong to the same organization. For example:

film.theguardian.com
music.theguardian.com

news.sussex.ac.uk
  blog.sussex.ac.uk
admissions.sussex.ac.uk