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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9  Name Systems and TCP/IP Name Registration and Name Resolution
           9  TCP/IP Name Systems: Host Tables and Domain Name System (DNS)
                9  TCP/IP Domain Name System (DNS)
                     9  DNS Name Space, Architecture and Terminology

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DNS Domains and the DNS Hierarchical Name Architecture
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DNS Labels, Names and Syntax Rules
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DNS Structural Elements and Terminology: Domains, Subdomains, and Nodes; Roots, Leaves and Branches; Parents, Children and Siblings
(Page 3 of 3)

DNS Family-Related Terminology

Another set of terminology you will run into compares the DNS tree structure not to a living tree, but in fact, to another analogy: a family tree. These terms are most often used to describe how a particular domain under discussion relates to the other domains or subdomains around it, so they are relative terms. The ones usually seen are (see Figure 237):

  • Parent Domain: The domain that is above this one in the hierarchy. For example, the root domain is the parent of all top-level domains.

  • Child: A domain at the next level down from this one in the hierarchy. Thus, the top-level domains are children of the root.

  • Sibling: A peer at the same level as this one in the hierarchy, with the same parent. Thus, all top-level domains are siblings with the root as a parent; all second-level domains within a particular TLD are siblings and so on.

    Figure 237: DNS Name Space “Family Tree”

    This diagram is similar to Figure 236, but I have labeled the nodes differently to show the “family-oriented” terminology sometimes used in DNS. In this case, the names are relative to the interior node shown in cyan. The domain immediately above it is its parent node; other nodes on the same level are siblings, and subdomains within it are children of that node.


Key Concept: The domain above a given domain in the DNS name space is called its parent domain; domains at the same level within the same parent are siblings; and subdomains are called children of that domain.

Restrictions on DNS Tree Structure

Note that like a real tree, the DNS name structure must be a true tree in its structure. Every domain can have only one parent (except the root), just as every branch of a tree connects to only one limb (except the root/trunk). Also, no “loops” can appear in the structure; you can't have a domain whose child is also its parent, for example.

Key Concept: A DNS name space must be arranged as a true topological tree. This means each domain can have only one parent, and no “loops” are permitted in the structure.

I also want to point out that even though the name hierarchy represents an arrangement of named devices, it is only a logical structure. There is no necessary correspondence to the physical location of devices. A domain with ten children may represent 11 devices in 11 different countries. We'll explore this more when we look at DNS authority structures.

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DNS Domains and the DNS Hierarchical Name Architecture
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Next Page
DNS Labels, Names and Syntax Rules
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Version 3.0 - Version Date: September 20, 2005

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