- TLD = Top-Level Domain
Whenever you use a domain name, in a web address(URL), email address, or wherever, it ends in a "top-level domain" or "TLD". This is the last part of the name. We often thing of .COM, .ORG, .NET, etc., as in:
(I've capitalized and made the TLDs bold here, but in the DNS case doesn't matter.)
TLDs are broadly classified into two categories:
- generic top-level domains (gTLDs)
- country code top-level domains (ccTLDs)
The entity responsible for the administration of these TLDs in the "root" of the Domain Name System (DNS) is the Internet Assigned Numbers Authority (IANA) that is currently operated by the Internet Corporation for Assigned Names and Numbers (ICANN). You can see the full list of current TLDs at:
- Second-Level Domain
The next part of the domain name to the left of the TLD (and separated by a dot) is the "second-level domain". These are the domains that you are typically able to register with a registrar. Examples include:
The next part of the domain name to the left ("www" in the first two examples above) would be called the "third-level domain", and so on.
- gTLD = Generic Top-Level Domain
Generic top-level domains (gTLDs) are TLDs that are not tied to any specific country and are "generic" in terms of being able to be used (in theory, anyway) by anyone on the Internet anywhere in the world. The "original" TLDs such as .COM, .ORG, .NET, .GOV, .MIL are all classifed as "generic TLDs". There were a couple of rounds of "expansion" of the gTLDs that brought the total to 22 gTLDs prior to the "newgTLD" expansion currently underway
- ccTLD = Country Code Top-Level Domain
Country code top-level domains(ccTLDs) are two letter TLDs that are assigned to countries based mostly on the ISOC 3166 list of country codes. Some countries have chosen to operate their ccTLD exclusively for domains within their country or geographic territory. Some do not allow people to register "second-level domains" under the TLD and instead require people to register third-level domains under one of several different second-level domains. For example, the .UK domain as to date required registrations to be under domains such as ".co.uk" and ".org.uk", basically duplicating part of the original gTLD scheme inside their ccTLD.
Many ccTLDs have chosen NOT to restrict their ccTLD to people in their country and have in fact marketed their domains very widely encouraging everyone to use them. Some prominent examples of this include Columbia(.CO), Montenegro(.ME), Tuvulu(.TV), Federated States of Micronesia(.FM) and many more.
Essentially, any time you are using a two-letter TLD, it is a ccTLD for some country. (With a few exceptions.)
- newgTLD = Top-Level Domain
After many years of discussion, ICANN's board voted in 2011 to allow the creation of new generic TLDs using almost any text string (and in multiple character sets) and began the "newgTLD" program. This resulted in 1,930 applications by various companies to operate new gTLDs. These newgTLDs are now being rolled out in phases and people are able to register second-level domains under many of these domains. More newgTLDs are being made available pretty much every week - and the expansion will continue for many months and years ahead of us. I recently wrote on CircleID about how to keep track of all the newgTLDs.
At a technical level, "new gTLDs" are effectively the same as "gTLDs"... the designation is just really that these new gTLDs are coming out in this current round of expansion.
- IDN = Internationalized Domain Name
The original TLDs were all in the ASCII character set, but over time ICANN decided to allow the creation of "internationalized domain names"(IDNs) that use other character sets such as Cyrillic, Arabic, Chinese, Korean, etc. The first IDN for a country code TLD appeared in 2010 and the newgTLDs contain many IDNs. (In fact, the very first of the "newgTLDs" were four IDNs.)
These are the basic terms you will hear for domain names when you are talking about the newgTLD program. There are a host of other issues, topics and discussion points that can be discussed in future posts... but I wanted to get these basic terms out there as a baseline.
Comments are, of course, welcome.
 - Some of these "generic" TLDs are now formally classified by ICANN as "sponsored" when the registration of domain names is restricted to a particular type of user. For example, registrations in .GOV are restricted to US government entities and registrations in .MUSEUM are restricted to museums. In these cases there is a sponsoring organization or government who manages the TLD.
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