Which of the following is a requirement for a fully qualified domain name (FQDN)?

A fully qualified domain name (FQDN) is essential for identifying a specific host within the hierarchical Domain Name System (DNS). To be considered fully qualified, it must include both the hostname and the domain name, which together provide an unambiguous and complete address for a resource on the internet.

The hostname is the specific name of the individual machine or service, while the domain name specifies the broader context, including the organization and the top-level domain (like .com, .org, etc.). For instance, in "mail.example.com," "mail" is the hostname, and "example.com" is the domain name. This structure ensures that the FQDN can be resolved to an IP address by DNS resolvers, leading to the correct resource.

Other options provided in the question do not fulfill the requirements of an FQDN. An IP address is typically not part of the FQDN itself, but rather the result of resolving the FQDN. Additionally, a subdomain and merely the name of a server do not represent the complete structure needed for an FQDN. Therefore, the requirement for a fully qualified domain name distinctly emphasizes the necessity of both the hostname and the domain name.