DNS on Windows Server 2003, 3rd Edition

Domain Names

Each node in the tree has a text label (without dots) that can be up to 63 characters long. A null (zero-length) label is reserved for the root. The full domain name of any node in the tree is the sequence of labels on the path from that node to the root. Domain names are always read from the node toward the root (“up” the tree), with dots separating the names in the path.

If the root node’s label actually appears in a node’s domain name, the name looks as though it ends in a dot, as in “www.oreilly.com.”. (It actually ends with a dot—the separator—and the root’s null label.) When the root node’s label appears by itself, it is written as a single dot, “.”, for convenience. Consequently, some software interprets a trailing dot in a domain name to indicate that the domain name is absolute. An absolute domain name is written relative to the root and unambiguously specifies a node’s location in the hierarchy. An absolute domain name is also referred to as a fully qualified domain name, often abbreviated FQDN. Names without trailing dots are sometimes interpreted as relative to some domain name other than the root, just as directory names without a leading slash are often interpreted as relative to the current directory.

DNS requires that sibling nodes—nodes that are children of the same parent—have different labels. This restriction guarantees that a domain name uniquely identifies a single node in the tree. The restriction really isn’t a limitation, because the labels ...