There is a hierarchy within the domain name registry that distinguishes domains from one another.
Top-Level Domains (TLD)
In 1985, The Internet Assigned Numbers Authority (IANA) released six top-level domain names. These top-level domains (TLD) became known as domain name extensions and represent the highest level in the Domain Name System hierarchy. They include:
- .com: shorthand for commercial, .com was the first top-level domain in common use. While .com was initially created for use by commercial organizations, restrictions on this were not stringent. By the mid-1990’s, .com had become the most popular and commonly used type of top-level domain for businesses, websites, and email.
- .net: shorthand for network, .net was created expressly for institutes that partook in network technologies such as an internet service provider or an infrastructure company. Like with .com, the restrictions meant to limit .net to networking purposes, was never upheld and it became one of the more popular top-level domains, with many seeing it as a close second to using the .com top level domain.
- .edu: shorthand for education, .edu was made for education institutions. Although it was intended for universities everywhere, the TLD .edu became associated with only educational centers in America. Schools from other countries will use .edu in conjunction with their country-level domain, which we will discuss in the next section below.
- .org: shorthand for organization, .org was created for nonprofits. As we’ve seen with these other top-level domains, such intentions were often not upheld or enforced over time. These days, .ors is used as top-level domain by nonprofits, for-profit businesses, schools, and communities.
- .mil: shorthand for military, .mil was created expressly for U.S. military branches. Unlike the other different types of top-level domains, this restriction is still upheld. Now, it is quite common for .mil to use second and third-level domains in conjunction with the .mil TLD.
- .gov: shorthand for government, .gov, like .mil, was restricted for American federal governmental agencies and personnel use only. These days, .gov is used by governmental agencies, programs, cities, states, towns, counties, and native American tribes.
Country code top-level domains (ccTLD)
In order to distinguish one country from another, especially one that would like to use a top level domain such as .gov or .mil, two letter domains were established and became associated with countries or geographical locations; .uk and .au, for example, to represent England and Australia, respectively. When initially created it was intended for registration to a corresponding ccTLD to be limited to that countries residents, however, certain countries have let outside parties register domain names using their country code.
Internationalized country code top-level domains (IDN ccTLD)
This was a top-level name with an encoded format that lets non-Latin character sets or other special characters be used.
Generic top-level domain (gTLD)
Generic top-level domains function as a category of top-level domains within the DNS. As of now, there are currently 21 generic top-level domains within the root zone, which is the highest level of the domain name system structure. While there are over 1,500 gTLDs in use, these 21 make up the vast majority of all types of domain names. They include four sub-categories:
- Generic (.com, .net, .org, .info), domains that can be used for general purposes.
- Generic restricted (.pro, .biz, .name) domains that can only be used for their specific purposes.
- Sponsored (.edu, .gov, .int, .mil, .aero, .cat, .asia, .mobi, .coop, .travel, .tel, .jobs) domains that can only be used by businesses involved specifically with that industry.
- Infrastructure (.arpa) which was one of the original top-level domains used to help with the DNS infrastructure.
Within the DNS hierarchy, second-level domains are domains that follow top-level domains. For example, in Nike.com, Nike is the second-level domain of the .com top-level domain. Quite often, second-level domains are the name of the business or vendor that registered the domain name with a registrar. The brand name, company name, or project name is the identifier for potential customers.
On top of these general second level domains, there are also country code second-level domains (ccSLD). In such cases, the second-level domain will be found to the right of the period; for example, in a domain such as nike.co.ca, the country code top-level domain is .ca and the ccSLD is .co.
Within the DNS hierarchy, third-level domains naturally follow second-level domains. They can be found to the left of SLD and are often referred to as the subdomain. Larger companies will often use third-level domains as identifiers that can distinguish between various departments. Generally speaking, “www” is the most common third-level domain. If a company does use multiple third-level domains, those are generally referring to a specific server within the company.
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