Skip to main content

Domain Name Format

Updated Jan 16, 2018 ·

Domain Name Format​

Domain names are interpreted by DNS from right to left, unlike how we type them in a browser (left to right).

For example, we type the address from left to right, starting with www:

## Typed from left to right -----> 
www.example.com

However, DNS resolves it starting from the rightmost part (com) and works its way left:

## <------ Resolved from "com", then to "example"...
www.example.com

DNS Hierarchy​

DNS is organized in a hierarchical structure:

In practice, we usually interact with domain names through URLs, like:

https://www.example.com

Components of a Domain Name​

  1. Root

    • A special dot (.) at the rightmost position.

    • Cannot be replaced; the dot is unique.

    • This is the top of the hierarchy

      info

      The trailing dot in a domain name, representing the root, is often invisible because it's implied and not necessary for most user applications.

      This is the reason why we write:

      www.cats.example.com

      and not:

      www.cat.example.com.

  2. Top-Level Domain (TLD)

    • Comes after the root (e.g., com, net, org, io).
    • Indicates the type of domain (commercial, organization, etc.).

  3. Second-Level Domain (SLD)

    • Example: The example in www.example.com.
    • SLD + TLD form the zone apex
    • Also called the naked or apex domain.

  4. Third-Level Domain

    • Usually www, but can be any label like bbb or abc.
    • Primarily a naming convention, not required.

Labels and Subdomains​

The components of a domain name are called labels, with the root label being null. Each label is a subdomain of its parent:

For example:

cats.example.com
  • cats is a subdomain of example.com
  • example is a subdomain of com

FQDN and PQDN​

The FQDN (Fully Qualified Domain Name) is the complete domain from root to specific subdomain. The PQDN (Partially Qualified Domain Name) starts with a hostname but may not include the root.

info

FQDN is not equal to the URL, as URLs include protocol and path (e.g., https://www.example.com/path).

Domain Name Syntax Rules​

  • Labels

    • Max 63 characters
    • Can use letters (A-Z), digits (0-9), and hyphen.
    • Cannot start or end with a hyphen.

  • Domains

    • TLDs cannot be all numeric.
    • Unlimited number of subdomains allowed.
    • Entire domain name: max 255 characters, including dots.