A top-level domain (TLD) is one of the domains at the highest level in the hierarchical Domain Name System of the Internet.
The top-level domain names are installed in the root zone of the name space. For all domains in lower levels, it is the last part of the domain name, that is, the last label of a fully qualified domain name.
For example, in the domain name www.example.com, the top-level domain is com. Responsibility for management of most top-level domains is delegated to specific organizations by the Internet Corporation for
Assigned Names and Numbers (ICANN), which operates the Internet Assigned Numbers Authority (IANA), and is in charge of maintaining the DNS root zone.
When an application requests services from TCP, UDP, or SCTP at the transport layer, it must supply a numerical IP address.
TCP/IP provides a support application that fulfills the role of the Domain Name System (DNS) protocol, to translate domain names into IP addresses.
The DNS application uses a massive distributed database organized as a directory system of servers to obtain the required information.
Each entry in the database consists of a domain name and an associated IP address.
The directory system that is used to translate domain names into IP addresses is organized as a tree structure, very similar to the directory structure of a computer operating system, except that there is a separate server at each node on the tree.
Figure 4-10 shows the structure of the tree. Each directory node on the tree provides name-to-IP address services corresponding to its
position on the tree. There are three primary levels of interest.
Below that, individual domain name owners can extend the number of levels down as far as they wish for convenience of organization and clarity.
At the top of the tree is the root directory, called the DNSroot server.
Actually, there are thirteen of these servers, scattered all over the world, and each of them is a cluster consisting of many computers. The DNS root servers must handle a large number of queries, more than 50 billion a day as of June 2009.
Disbursing the root servers geographically reduces the amount of long distance traffic by providing nearby access for as many queries as possible. The DNS root servers have entries
for all of the so-called top-level domains.
There are country-code top-level domain name servers (ccTLDs) for every identifiable country in the world plus a number of authorized
commercial and noncommercial type domains. The non-country-code domains include .com, .edu, .org, .net, and many more.
In mid-2008, a decision was made to allow the
creation of additional top-level domains. As of June, 2008, there were more than 160 million domain name registrations, according to the Domain Name Industry Brief [Domain2008].
Domain names below the top-level domains are registered for a small fee by users with one of a number of registrars. ICANN assumes overall
responsibility for the millions of registered names on the Internet. Domain names at this level are called local domains. The name must, of course, be unique; there can be no
duplicates anywhere in the world. Once the domain name is registered, it can be used to assign names of individual nodes or hosts within that domain, and matched to one or more
assigned IP addresses. Domain names are read left to right, from the lowest subdomain to the top level domain.