Domain Name Space

Domain Name Space

The domain name space contains a tree of domain names that uses the Hierarchical name space. The node in the tree stores the information of the associated domain name. Domain name space is based on an inverted tree structure with the root at its top is shown in Starting from 0 (root level) to 127 (highest level), a tree can have up to 128 levels. The domain names in DNS are not case-sensitive; for example, express is treated the same as Express or EXPRESS.

Domain names are sequentially arranged labels that start from the current node up to the root. The label of the root is null, which is also the last node. Sequenced labels are separated using dots.

A domain name can be of two types- Fully Qualified Domain Name (FQDN) and Partially Qualified Domain Name (PQDN). The following are a brief description of FQDN and PQDN:

  • FQDN: Defines all domain levels of DNS tree hierarchy consisting of the root domain.
  • PQDN: Specifies domain levels from the current node in the tree hierarchy of DNS, but does not end with the root domain.

Domain name space uses some basic terms and concepts, such as domains, zones, and root name server.


Domain refers to sub-trees in the domain name space. The name of the domain is the top node of the subtree. From Fig.NET-4.7, cam, edu, gov, and mil are top level domains. Also, from the subtree of Microsoft is the domain of this subtree.


The management of database in DNS is not an easy task, because of its large size. Therefore, the complete hierarchy of domain names cannot be stored on a single server. To manage domain names, the work of the main server is divided into several servers. Due to this, entire name space is divided into zones. A zone; therefore, is an area controlled by a particular server, which is under the main server, in the hierarchy. There are two types of zones- primary and secondary. The primary zone is a master copy and secondary zone is a read-only copy of the zone.

Root Name Server

A server that has central authority over all other servers in the hierarchy of the tree is called the Root name server. It does not store information related to domains; rather, it provides addresses of DNS servers controlling different top-level domains and zones. The Root name server is of two types- primary and secondary.

The Primary Name Server

The primary name server manages zonal files (files of a zone), on which it has authority and store these files on a local hard disk. This server is responsible for creating, maintaining, and updating zonal files.

The Secondary Name Server

The secondary name server transfers complete information about a zone from another (primary or secondary) server. In addition, the secondary name server stores files on local hard disks, but these servers can neither create nor update zonal files. Both the primary and secondary name servers are responsible for their zones. The secondary name server does not create or update zonal files when the primary name server crashes. The secondary name server stores those files as a primary name server backup. Let’s now learn about the resolver.


A resolver is a DNS client that maps a domain name to the IP address and vice-versa. A host calls the resolver, which accesses the closest DNS server for mapping a request. If the DNS server has the information of the domain name, then resolver sends back the information to the requested client; otherwise, the resolver sends the request to another DNS server. With this, we come to the end of-the chapter; let’s summarize the key point discussed in the chapter.


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