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How the Domain namespace is organized


rev.dennis
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Any DNS domain name used in the tree is technically a domain. Most DNS discussions, however, identify names in one of five ways, based on the level and the way a name is commonly used. For example, the DNS domain name registered to HOSANG I.T. (hosangit.com.) is known as a second-level domain. This is because the name has two parts (known as labels) that indicate it is located two levels below the root or top of the tree. Most DNS domain names have two or more labels, each of which indicates a new level in the tree. Periods are used in names to separate labels.
 
The five categories used to describe DNS domain names by their function in the namespace are described in the following table, along with an example of each name type.

Name Type Description Example
The domain root This is the top of the tree, representing an unnamed level; it is sometimes shown as two empty quotation marks (""), indicating a null value. When used in a DNS domain name, it is stated by a trailing period (.) to designate that the name is located at the root or highest level of the domain hierarchy. In this instance, the DNS domain name is considered to be complete and points to an exact location in the tree of names. Names stated this way are called fully qualified domain names (FQDNs). A single period (.) or a period used at the end of a name, such as "example.hosangit.com.".
Top-level domain A name of two or three letters used to indicate a country/region or the type of organization using a name. For more information, see Top-level domains. ".com", which indicates a name registered to a business for commercial use on the Internet.
Second-level domain Variable-length names registered to an individual or organization for use on the Internet. These names are always based upon an appropriate top-level domain, depending on the type of organization or geographic location where a name is used. "hosangit.com.", which is the second-level domain name registered to HOSANG I.T. by the Internet DNS domain name registrar.
Subdomain Additional names that an organization can create that are derived from the registered second-level domain name. These include names added to grow the DNS tree of names in an organization and divide it into departments or geographic locations. "example.hosangit.com.", which is a fictitious subdomain assigned by HOSANG I.T. for use in documentation example names.
Host or resource name Names that represent a leaf in the DNS tree of names and identify a specific resource. Typically, the leftmost label of a DNS domain name identifies a specific computer on the network. For example, if a name at this level is used in a host (A) RR, it is used to look up the IP address of computer based on its host name. "host-a.example.hosangit.com.", where the first label ("host-a") is the DNS host name for a specific computer on the network.

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