DHCP (Dynamic Host Configuration Protocol) is the service used to automatically assign IP addresses to network hosts. It dynamically manages the distribution of IP addresses and configuration details (like subnet mask, gateway, and DNS servers), ensuring no two devices are assigned the same address on the network.

Option B (DNS): Incorrect. DNS translates domain names into IP addresses but does not assign them.

Option C (LDAP): Incorrect. LDAP is a protocol used for accessing and maintaining directory services, such as user information and permissions.

Option D (SMTP): Incorrect. SMTP is used for sending and receiving email, not for IP address assignment.