Next are third-level domains, which are written immediately to the left of a second-level domain. There can be fourth- and fifth-level domains, and so on, with virtually no limitation. An example of an operational domain name with four levels of domain labels is www.sos.state.oh.us. The www preceding the domains is the host name of the World-Wide Web server. Each label is separated by a full stop (dot). 'sos' is said to be a sub-domain of 'state.oh.us', and 'state' a sub-domain of 'oh.us', etc. In general, subdomains are domains subordinate to their parent domain. An example of very deep levels of subdomain ordering are the IPv6 reverse resolution DNS zones, e.g., 1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.ip6.arpa, which is the reverse DNS resolution domain name for the IP address of a loopback interface, or the localhost name.
Second-level (or lower-level, depending on the established parent hierarchy) domain names are often created based on the name of a company (e.g., microsoft.com), product or service (e.g., gmail.com). Below these levels, the next domain name component has been used to designate a particular host server. Therefore, ftp.wikipedia.org might be an FTP server, www.wikipedia.org would be a World Wide Web server, and mail.wikipedia.org could be an email server, each intended to perform only the implied function. Modern technology allows multiple physical servers with either different (cf. load balancing) or even identical addresses (cf. anycast) to serve a single hostname or domain name, or multiple domain names to be served by a single computer. The latter is very popular in Web hosting service centers, where service providers host the websites of many organizations on just a few servers.
Allowed character set
Domain name registrations have traditionally only been allowed for names that consist only of letters, digits and the hyphen (-) from the ASCII character set, as in hostnames. The full stop (dot, .) is used to separate DNS labels, the hierarchical components in a domain name.
This character set excludes numerous characters commonly found in non-English languages, and does not allow multi-byte characters necessary for most Asian languages. The Internationalized domain name (IDN) system, which permits such characters, has been developed and many registries allow such names.
The underscore character is permitted in names used in the domain name system, and is frequently used to ensure that a domain name is not recognized as a hostname, as in the use of DNS server records (SRV), for example. Other naming systems often used in conjunction with DNS, such as NetBIOS, allow it.
Official assignment
The Internet Corporation for Assigned Names and Numbers (ICANN) has overall responsibility for managing the DNS. It administers the root domain, delegating control over each TLD to a domain name registry. For ccTLDs, the domain registry is typically installed by the government of that country. ICANN has a consultation role in these domain registries but cannot regulate the terms and conditions of how domain names are delegated in each of the country-level domain registries. On the other hand, the generic top-level domains (gTLDs) are governed directly under ICANN, which means all terms and conditions are defined by ICANN with the cooperation of each gTLD registry.
Domain names are often seen in analogy to real estate in that (1) domain names are foundations on which a website (like a house or commercial building) can be built and (2) the highest "quality" domain names, like sought-after real estate, tend to carry significant value, usually due to their online brand-building potential, use in advertising, search engine optimization, and many other criteria.
A few companies have offered low-cost, below-cost or even cost-free domain registrations with a variety of models adopted to recoup the costs to the provider. These usually require that domains be hosted on their website within a framework or portal that includes advertising wrapped around the domain holder's content, revenue from which allows the provider to recoup the costs. Domain registrations were free of charge when the DNS was new. A domain holder (often referred to as a domain owner) can give away or sell infinite number of subdomains under their domain name. For example, the owner of example.edu could provide subdomains such as foo.example.edu and foo.bar.example.edu to interested parties.
Within a particular TLD, parties are generally free to register an undelegated domain name on a first come, first served basis, resulting in Harris's lament, all the good ones are taken. For generic or commonly used names, this may sometimes lead to the use of a domain name which is inaccurate or misleading. This problem can be seen with regard to the ownership or control of domain names for a generic product or service. By way of illustration, there has been tremendous growth in the number and size of literary festivals around the world in recent years. In the current context, a generic domain name such as literary.org is available to the first literary festival organization that is able to obtain the registration, even if the festival in question is very young or obscure. Some critics argue that there is greater amenity in reserving such domain names for the use of, for example, a regional or umbrella grouping of festivals. Related issues may also arise in relation to noncommercial domain names.
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