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DNS Labels, Names and Syntax Rules
(Page 1 of 3)
We've seen how the DNS name space
hierarchy allows us to arrange domains into a virtual tree that reflects
the characteristics of how the devices themselves are organized. While
using a hierarchical name space is inherently more complex than a flat
name space, it yields a powerful result: the ability to specify names
that can be locally managed while remaining globally unique. At the
same time, the complexity of the tree yields the benefit of relatively
simple name construction using domain identifiers.
DNS Labels and Label Syntax Rules
Naming in DNS begins with giving
each domain, or node, in the DNS name space a text label. The
label identifies the domain within the structure, and must follow these
DNS Label Uniqueness Requirement
- Length: Each label can theoretically be
from 0 to 63 characters in length. In practice, a length of 1 to about
20 characters is most common, with a special exception for the label
assigned to the root of the tree (see below).
- Symbols: Letters, numbers are allowed,
as well as the dash symbol (-). No other punctuation is
permitted, including the underscore (_).
- Case: Labels are not case-sensitive. This
means that Jabberwocky and jabberwocky are both
permissible domain name labels, but they are equivalent.
Every label must be unique within
its parent domain. So, for example, if we have a top-level domain
(TLD) called rocks, we can only have one subdomain within
rocks called crystal. Due to the case insensitivity
of labels, we cannot have both CRYSTAL and crystal
within rocks, because they are considered the same.
It is this concept of local
uniqueness within a parent domain that ensures the uniqueness
of names as a whole, while allowing local control over naming. Whoever
is in charge of the rocks domain can assign names to as
many individual objects or subdomains as he likes as long as they are
unique within the domain. Someone else, say, the maintainer of the glass
domain, can also create a subdomain called crystal within
glass; there is no conflict because the glass
and rocks domains are separate. Of course, since all top-level
domains have the same parent (the root), all TLDs must be unique.
Key Concept: Each node in the DNS name space is identified by a label. Each label must be unique within a parent domain, but need not be unique across domains. This enables each domain to have local control over the names of subdomains without causing any conflict in the full domain names created on a global level.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
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