A telephone company is responsible for maintaining a list of 25 million names and phone numbers for inclusion in printed phone books as well as in an inline phone directory accessible on the…

A web server delivers HTTP content and pages to browsers but is not designed to store or manage structured datasets of 25 million records.

Directory servers implement protocols like LDAP and are specifically optimized for storing large hierarchical datasets of named entries - exactly the structure of a phone directory. They are designed for high-volume read queries such as name-to-number lookups, and serve both internal systems and public-facing applications like an online phone directory simultaneously.

A file server stores and shares unstructured files across a network and lacks the query and indexing capabilities needed for fast lookups in a large structured directory.

While a database server can store structured data, directory servers are the more appropriate and specialized choice specifically for named-attribute directory lookups like telephone directories.