Which of the following server types stores the list of network usernames and passwords required for authentication?

The directory server is specifically designed to store and manage the information related to network resources, including usernames and passwords required for authentication. In many network environments, directory services such as Microsoft's Active Directory or Lightweight Directory Access Protocol (LDAP) are utilized to maintain this centralized repository of user identities. These services not only hold the credentials for users but also manage permissions and policies that govern access to various network resources.

File servers, on the other hand, primarily function to facilitate file storage and sharing across a network, not to handle authentication credentials. While they may interact with directory services to validate users accessing files, they do not inherently store usernames and passwords themselves.

DHCP (Dynamic Host Configuration Protocol) servers are responsible for assigning IP addresses and other network configuration parameters to devices in a network, enabling them to communicate effectively. They do not deal with user authentication or the management of usernames and passwords.

SNMP (Simple Network Management Protocol) is utilized for network management and monitoring. It helps in overseeing network devices and their performance but does not pertain to user credentials or authentication processes.

Thus, the directory server is the correct choice as it directly relates to the management and storage of usernames and passwords needed for network authentication.