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Second Level Domain - SLD

The SLD (Second-Level Domain) is the part of a domain name that appears directly to the left of the Top-Level Domain (TLD).

Example:

In the domain
👉 example.com

  • .com is the TLD (Top-Level Domain)

  • example is the SLD (Second-Level Domain)


Domain Structure (from right to left):

Level Example
Top-Level Domain .com
Second-Level Domain example
Subdomain (optional) www. or e.g. blog.

More Examples:

Domain SLD TLD
google.de google .de
wikipedia.org wikipedia .org
meinshop.example.com example .com

Why it matters:

The SLD is usually the custom, chosen part of the domain—often representing a company name, brand, or project. It's the most recognizable and memorable part of a domain name.

 


Fully Qualified Domain Name - FQDN

The Fully Qualified Domain Name (FQDN) is the complete and unique name of a computer or host on the internet or a local network. It consists of multiple parts that reflect a hierarchical structure.

Structure of an FQDN

An FQDN is made up of three main components:

  1. Hostname – The specific name of a computer or service (e.g., www).

  2. Domain Name – The name of the higher-level domain (e.g., example).

  3. Top-Level Domain (TLD) – The highest level of the domain structure (e.g., .com).

Example of an FQDN:
👉 www.example.com.

  • www → Hostname

  • example → Domain name

  • .com → Top-Level Domain

  • The trailing dot (.) is optional and represents the root domain of the DNS system.

Why is the FQDN important?

Uniqueness: Each FQDN is globally unique and refers to a specific resource on the internet.
DNS Resolution: It is used by DNS servers to find the IP address of websites and servers.
SSL Certificates: An FQDN is often required for SSL/TLS certificates to ensure secure connections.
Email Delivery: Mail servers use FQDNs to send emails to the correct hosts.

Difference Between an FQDN and a Simple Domain

  • FQDN: mail.google.com (fully specified)

  • Simple Domain: google.com (can contain multiple hosts, e.g., www, mail, ftp)

In summary, the FQDN is the complete address of a device or service on the internet, while a simple domain is a more general address.


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