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DNS Structural Elements and Terminology: Domains, Subdomains, and Nodes; Roots, Leaves and Branches; Parents, Children and Siblings
(Page 1 of 3)
Now that we understand the fundamentals
of the Domain Name System's name space, let's look at the DNS structure
in more detail. At the same time, we can get a better handle on the
many different terms used to refer to parts of the DNS domain name hierarchy.
DNS Tree-Related Terminology
We saw in the
previous topic that the DNS name structure
is shaped somewhat like a tree; the comparison between structured elements
and trees is a common one in networking. The main difference between
technology and biology here is that DNS trees grow from the top down,
instead of reaching for the sky. The analogy to a tree naturally leads
to the use of several tree-related terms in describing the DNS name
structure, some of which are illustrated in Figure 236:
- Root: This is the conceptual top of the
DNS name structure. The root domain in DNS contains the entire structure.
By definition, it has no name; it is null.
- Branch: A branch is any contiguous portion
of DNS hierarchy. It consists of a domain and all the domains and objects
within it. All branches connect together to the root, just like in a
real tree. (Yes, it would be better if the root were called the trunk,
but computer science majors apparently don't take botany as electives.
- Leaf: This is an end object
in the structure, that is, a domain that doesn't have anything underneath
it. Again, the analogy to a leaf being at the end of a sequence of branches
There is no specific term to refer
to a domain that is not a leaf. These are sometimes called interior
nodes, meaning that they are in the middle of the structure. A node
is the generic computing term for an object in a topology or structure;
it is used throughout this Guide. So, in DNS, every node is a domain,
and may be either an interior node that contains additional domains
and/or objects, or a leaf that is a specific named device. The term
domain is thus somewhat ambiguous, as it can refer to either
a collection of objects which represents a branch of the tree, or a
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Version 3.0 - Version Date: September 20, 2005
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