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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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Version 3.0 - Version Date: September 20, 2005
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