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

Now that we understand the fundamentals of the Domain Name System's name space, let's look at the DNS structure in more detail. At the same time, we can get a better handle on the many different terms used to refer to parts of the DNS domain name hierarchy.

We saw in the previous topic that the DNS name structure is shaped somewhat like a tree; the comparison between structured elements and trees is a common one in networking. The main difference between technology and biology here is that DNS trees grow from the top down, instead of reaching for the sky. The analogy to a tree naturally leads to the use of several tree-related terms in describing the DNS name structure, some of which are illustrated in Figure 236:

• Root: This is the conceptual top of the DNS name structure. The root domain in DNS contains the entire structure. By definition, it has no name; it is null.
  
  
• Branch: A branch is any contiguous portion of DNS hierarchy. It consists of a domain and all the domains and objects within it. All branches connect together to the root, just like in a real tree. (Yes, it would be better if the root were called the trunk, but computer science majors apparently don't take botany as electives. J)
  
  
• Leaf: This is an “end object” in the structure, that is, a domain that doesn't have anything underneath it. Again, the analogy to a leaf being at the end of a sequence of branches is apt.

There is no specific term to refer to a domain that is not a leaf. These are sometimes called interior nodes, meaning that they are in the middle of the structure. A node is the generic computing term for an object in a topology or structure; it is used throughout this Guide. So, in DNS, every node is a domain, and may be either an interior node that contains additional domains and/or objects, or a leaf that is a specific named device. The term domain is thus somewhat ambiguous, as it can refer to either a collection of objects which represents a branch of the tree, or a specific leaf.

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