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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 Domain-Related Terminology

There are also several “domain-like” terms that are often used to refer to domains at different levels of the hierarchy. These terms, shown in Figure 236, include:

• Root Domain: This is the root of the tree, as before.
  
  
• Top-Level Domains (TLDs): These are the highest-level domains directly under the root of the tree. They are also sometimes called first-level domains.
  
  
• Second-Level Domains: Shockingly enough, these are the domains located directly below the top-level domains.
  
  
• Subdomains: In some contexts, this term refers only to domains that are located directly below the second-level domains (but see below).
  

  Figure 236: DNS Name Space Tree and Domain-Related Terminology

  The top of the DNS name space is the root of the tree, and has no name. Under the root comes any number of top-level domains (TLDs); within each of these can be placed second-level domains, then within those subdomains, and so forth.

  Some of the “tree” terminology used in DNS is also shown here. The portion of the tree with the purple background is one branch; the blue area highlights a smaller “sub-branch” within that branch. The green nodes within that blue area are the leaves of that smaller branch of the tree.

  
  

The term “subdomain” can also be used generically, like the word “domain” itself. In that case, it refers simply to the relationship between two domains, with a subdomain being under another domain in the structure. This means, for example, that top-level domains can be said to be subdomains of the root; every second-level domain is a subdomain of a top-level domain, and so on. But again, sometimes “subdomain” means specifically a third-level or lower domain.

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