Domain Name Format and DNS Hierarchy

A valid domain name meets the following requirements:

• A domain name is segmented using periods (.) into multiple labels.
• A domain name label can contain letters, digits, and hyphens (-) and cannot start or end with a hyphen.
• A label cannot exceed 63 characters.
• The total length of a domain name, including the period at the end, cannot exceed 254 characters.

A domain name is divided into the following levels based on its structure:

• Root domain: . (a period)
• Top-level domain: for example, .com, .net, .org, and .cn
• Second-level domain: subdomains of the top-level domain names, such as example.com, example.net, and example.org
• Third-level domain: subdomains of the second-level domain names, such as abc.example.com, abc.example.net, and abc.example.org
• The next-level domain names are similarly expanded by adding prefixes to the previous-level domain names, such as def.abc.example.com, def.abc.example.net, and def.abc.example.org.