A valid domain name meets the following requirements:
- A domain name is segmented using dots (.) 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 dot at the end, cannot exceed 254 characters.
Domain names are classified into the following levels based on their structure:
- Root domain: . (a dot)
- 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.