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

DNS Structural Elements and Terminology: Domains, Subdomains, and Nodes; Roots, Leaves and Branches; Parents, Children and Siblings 1 2 3 Absolute (Fully-Qualified) and Relative (Partially-Qualified) Domain Name Specifications

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 information—we 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.

DNS Structural Elements and Terminology: Domains, Subdomains, and Nodes; Roots, Leaves and Branches; Parents, Children and Siblings 1 2 3 Absolute (Fully-Qualified) and Relative (Partially-Qualified) Domain Name Specifications