The DNS structure contains a hierarchy of names with highest level as Root, and is unnamed. Top Level Domains (TLDs) are divided into classes based on rules that have evolved over time. Most TLDs have been delegated to individual country managers, whose codes are assigned from a table maintained by an agency of the United Nations. These are called country-code Top Level Domains, or ccTLDs. In addition, there are a limited number of "generic" Top Level Domains (gTLDs), which do not have a geographic or country designation.