220-1101 Question 67
Single answer137-139 Network Basic Input/Output System (NetBIOS)/NetBIOS over TCP/IP (NetBT)A technician is troubleshooting a small office file-sharing issue between two legacy Windows PCs on the same subnet. Users report that they can ping each computer by IP address, but browsing to shared folders by computer name fails. The network does not use DNS for these legacy systems, and the technician suspects NetBIOS name resolution traffic is being blocked by a recently updated host firewall. Which port should the technician allow first to restore NetBIOS name resolution by computer name?
- A
TCP 139
- B
UDP 137
- C
UDP 138
- D
TCP 445
Show answer and explanation
Correct answer: B
Explanation
The key clue is that connectivity by IP works, but access by computer name does not, in an environment relying on legacy NetBIOS rather than DNS. NetBIOS over TCP/IP (NetBT) uses UDP 137 for NetBIOS Name Service, UDP 138 for Datagram Service, and TCP 139 for Session Service. In this scenario, the most appropriate first port to check is UDP 137 because it handles name registration and name queries. TCP 445 is associated with SMB over TCP and is common in modern Windows networking, but it does not provide NetBIOS name resolution. This aligns with Microsoft networking documentation and standard port references for NetBT troubleshooting.
- A. Incorrect.
TCP 139 is used for NetBIOS Session Service, which supports session-oriented services such as file and printer sharing over NetBIOS. While this port may be involved in legacy SMB communication, it is not the primary port used for NetBIOS name resolution. A technician might choose this because it is strongly associated with legacy Windows sharing, but the scenario specifically points to name resolution failure.
- B. Correct.
UDP 137 is correct because NetBIOS Name Service uses port 137 for NetBIOS name registration and name resolution. If systems can communicate by IP but cannot resolve each other's computer names in a legacy NetBIOS-based environment, allowing UDP 137 is the most direct first step.
- C. Incorrect.
UDP 138 is used for NetBIOS Datagram Service, which supports connectionless communication such as browser announcements and some legacy messaging functions. It is related to NetBIOS traffic, so it is a plausible distractor, but it is not the primary port for resolving a hostname to an IP address.
- D. Incorrect.
TCP 445 is used by Microsoft-DS for SMB directly over TCP/IP without NetBIOS. Modern Windows file sharing commonly uses this port instead of NetBIOS ports 137-139. However, the scenario explicitly states these are legacy systems relying on NetBIOS name resolution, so TCP 445 does not address the immediate name-resolution problem.