Overview of DNS Hierarchy

Introduction #

The Domain Name System (DNS) hierarchy is a fundamental concept for understanding how domain names are structured and resolved. This documentation provides a detailed overview of the DNS hierarchy and its various components, and explains how they work together to enable domain name resolution on the internet.

The Hierarchical Structure #

DNS is organized in a hierarchical structure, which is often depicted as an upside-down tree, known as the DNS namespace. At the top of the hierarchy is the root, followed by Top-Level Domains (TLDs), Second-Level Domains (SLDs), and subdomains, progressively becoming more specific as you move down the tree.

1. The Root #

At the very top of the DNS hierarchy is the root. The root is essentially the starting point for resolving domain names. It doesn’t have a name but is sometimes represented by a dot (.).

There are a number of root servers globally (13 logical root server systems), and they are authoritative for the root zone. These servers know how to direct queries toward the TLD name servers.

2. Top-Level Domains (TLDs) #

Immediately below the root in the hierarchy are the Top-Level Domains (TLDs). TLDs are the rightmost part of a domain name. Examples include:

  • Generic TLDs (gTLDs): .com, .net, .org, .info
  • Country Code TLDs (ccTLDs): .us, .uk, .ca, .in
  • Sponsored TLDs (sTLDs): .gov, .edu, .mil
  • Newer gTLDs: .xyz, .shop, .app, .cloud

3. Second-Level Domains (SLDs) #

Below TLDs are the Second-Level Domains (SLDs). This is what most people think of as the actual domain name. For example, in example.com, “example” is the second-level domain. SLDs are unique within their parent TLD, and individuals and organizations can register them.

4. Subdomains #

Below second-level domains, you can have additional lower levels called subdomains. Subdomains can be used to organize different sections or services of a website. For example, blog.example.com or shop.example.com

Subdomains can themselves have subdomains (e.g., dev.blog.example.com), leading to a theoretically infinite hierarchy, although practical limits are imposed by various constraints.

The Resolution Process and Hierarchy #

The hierarchical structure of DNS is critical for the domain name resolution process, where a domain name is translated into an IP address. This process typically involves multiple queries through the hierarchy:

  1. Root Servers: The resolver sends a query to one of the root servers. The root server responds with a referral to the TLD servers for the requested TLD.
  2. TLD Servers: The resolver then queries one of the TLD servers, which respond with a referral to the authoritative name servers for the requested second-level domain.
  3. Authoritative Name Servers: Finally, the resolver sends a query to the authoritative name servers for the domain, which respond with the IP address for the domain or subdomain.

Delegation and Zones #

The DNS hierarchy is divided into zones. A zone is a portion of the DNS namespace over which an entity (often a domain registrar or hosting service) has control. Through a process called delegation, control of subdomains can be delegated to other entities by pointing to different name servers.

Conclusion #

The DNS hierarchy is an essential component of the Domain Name System, providing a scalable and distributed framework for domain name resolution. It ensures that domain names can be organized logically and resolved efficiently. The hierarchical structure, consisting of root, TLDs, SLDs, and subdomains, is critical to understanding how DNS functions. Through delegation and zoning, DNS hierarchy also allows for distributed management and control over different portions of the DNS namespace.

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