TCP ports 137-139 are used by which service?

• A. Hypertext Transfer Protocol (HTTP)
• B. File Transfer Protocol (FTP)
• C. Simple Mail Transfer Protocol (SMTP)
• D. NetBIOS

Answer: (D)

The concept here is how well-known ports map to specific network services. Ports 137–139 are assigned to NetBIOS over TCP/IP. NetBIOS provides naming, sessions, and datagram services on Windows networks, with 137 for name service (UDP), 138 for datagram service (UDP), and 139 for the session service (TCP). In contrast, HTTP uses port 80 (and 443 for HTTPS), FTP uses 20/21, and SMTP uses 25, so those services do not correspond to ports 137–139. Therefore, the service associated with those ports is NetBIOS.