The Public Suffix List is a cross-vendor initiative to provide an accurate list of domain name suffixes, maintained by the hard work of Mozilla volunteers and by submissions from registries, to whom we are very grateful. site 
A "public suffix" is one under which Internet users can (or historically could) directly register names. Some examples of public suffixes are .com, .co.uk and pvt.k12.ma.us. The Public Suffix List is a list of all known public suffixes.