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 Domains and the DNS Hierarchical Name Architecture 1 2 3 DNS Labels, Names and Syntax Rules

DNS Family-Related Terminology

Another set of terminology you will run into compares the DNS tree structure not to a living tree, but in fact, to another analogy: a family tree. These terms are most often used to describe how a particular domain under discussion relates to the other domains or subdomains around it, so they are relative terms. The ones usually seen are (see Figure 237):

• Parent Domain: The domain that is above this one in the hierarchy. For example, the root domain is the parent of all top-level domains.
  
  
• Child: A domain at the next level down from this one in the hierarchy. Thus, the top-level domains are children of the root.
  
  
• Sibling: A peer at the same level as this one in the hierarchy, with the same parent. Thus, all top-level domains are siblings with the root as a parent; all second-level domains within a particular TLD are siblings and so on.
  

  Figure 237: DNS Name Space “Family Tree”

  This diagram is similar to Figure 236, but I have labeled the nodes differently to show the “family-oriented” terminology sometimes used in DNS. In this case, the names are relative to the interior node shown in cyan. The domain immediately above it is its parent node; other nodes on the same level are siblings, and subdomains within it are children of that node.

  
  

Note that like a real tree, the DNS name structure must be a true tree in its structure. Every domain can have only one parent (except the root), just as every branch of a tree connects to only one limb (except the root/trunk). Also, no “loops” can appear in the structure; you can't have a domain whose child is also its parent, for example.

I also want to point out that even though the name hierarchy represents an arrangement of named devices, it is only a logical structure. There is no necessary correspondence to the physical location of devices. A domain with ten children may represent 11 devices in 11 different countries. We'll explore this more when we look at DNS authority structures.

DNS Domains and the DNS Hierarchical Name Architecture 1 2 3 DNS Labels, Names and Syntax Rules