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.

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 syntax rules:

• 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.