DNS Labels, Names and Syntax Rules
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Constructing Domain Names From Domain Labels
Each individual domain within the domain name structure can be uniquely identified using the sequence of labels that starts from the root of the tree and progresses down to that domain. The labels at each level in the hierarchy are listed in sequence, starting with the highest level, from right to left, separated by dots. The result is the formal definition of a domain name.
The root of the name space is given a zero-length, null name by default. That is, the label for the root exists, it's just empty. This little bit of geekery is done because the root technically is part of every domain name. This means it must be included in every domain name. If it were something long like root, we would have to include that at the end of every domain name. This would simply make every name longer, while not really adding any useful informationwe already know every domain name is under the root.
Let's take the example above again, of a top-level domain called rocks, within which is a second-level domain crystal. The domain name of rocks is rocks., with the dot separating rocks and (the null root). In practice, the trailing dot is often omitted, so the domain name of the TLD rocks can be considered to be just rocks. The subdomain crystal within rocks has the domain name crystal.rocks. If we had a named device called salt within the crystal.rocks domain, it would be called salt.crystal.rocks. This is pretty straight-forward, as you can see in the very simple Figure 238.
We can use these names to easily identify subdomains of a particular domain. For example, if we start with salt.crystal.rocks, it's obvious that crystal.rocks is its parent domain. It's also clear that both crystal.rocks and salt.crystal.rocks are subdomains of rocks; one is a single level down from rocks and the other is two levels down.
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Version 3.0 - Version Date: September 20, 2005
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