Which port and protocol is commonly used for LDAP directory services?

Which port and protocol is commonly used for LDAP directory services?

• A. 53 UDP
• B. 389 TCP
• C. 21 TCP
• D. 389 UDP

Answer: (B)

LDAP uses a directory access protocol that runs over TCP, with the standard, widely used port being 389. This combination—TCP on 389—is chosen because LDAP operations like bind, search, and modify rely on reliable, ordered delivery of requests and responses, which TCP provides. UDP is connectionless and does not guarantee delivery, making it unsuitable for the reliability requirements of directory services. The other options point to different services: 53 UDP is DNS, 21 TCP is FTP, and while LDAP can be run over UDP in some rare cases, the common, standard deployment uses TCP on 389. For secure LDAP, the parallel variant uses port 636, but the non-secure standard remains TCP on 389.

LDAP uses a directory access protocol that runs over TCP, with the standard, widely used port being 389. This combination—TCP on 389—is chosen because LDAP operations like bind, search, and modify rely on reliable, ordered delivery of requests and responses, which TCP provides. UDP is connectionless and does not guarantee delivery, making it unsuitable for the reliability requirements of directory services. The other options point to different services: 53 UDP is DNS, 21 TCP is FTP, and while LDAP can be run over UDP in some rare cases, the common, standard deployment uses TCP on 389. For secure LDAP, the parallel variant uses port 636, but the non-secure standard remains TCP on 389.