220-1101 Question 359
Single answerNetworkA technician is troubleshooting a small office network. Users report that they can reach the company file server at 192.168.10.20 and other devices on the local subnet, but they cannot browse external websites. On one affected Windows PC, the technician confirms the IP address is 192.168.10.55, the subnet mask is 255.255.255.0, and the DNS server is 8.8.8.8. A ping to 192.168.10.20 succeeds, but a ping to 8.8.8.8 fails. Which of the following is the MOST likely cause of the problem?
- A
The default gateway is missing or configured incorrectly on the PC
- B
The NIC duplex setting is mismatched between the PC and the switch
- C
The file server is not responding to SMB requests
- D
The PC has an incorrect subnet mask and should use 255.255.0.0
Show answer and explanation
Correct answer: A
Explanation
The key troubleshooting clue is that the PC can reach local devices but cannot reach an external IP address. That means DNS is not the primary issue, because the test to 8.8.8.8 uses a direct IP address and does not require name resolution. In TCP/IP networking, a host uses its subnet mask to determine whether a destination is local or remote. Remote traffic must be sent to the configured default gateway, typically the router interface on that subnet. If the default gateway is missing or incorrect, the host can still communicate locally but cannot reach the internet or other remote networks. This aligns with standard TCP/IP troubleshooting practices documented by Microsoft and common networking best practices used in CompTIA A+ objectives: verify IP address, subnet mask, default gateway, and DNS settings, then test local connectivity before testing remote connectivity.
- A. Correct.
This is correct. The PC can communicate with hosts on its local subnet, which indicates the IP address and subnet mask are sufficient for local traffic. However, failure to reach 8.8.8.8 shows traffic is not leaving the local network. The default gateway is required to route packets to remote networks, including the internet. If it is missing or incorrect, local resources remain reachable while off-subnet destinations fail.
- B. Incorrect.
This is incorrect. A duplex mismatch can cause poor performance, collisions, retransmissions, or intermittent connectivity, but it would not typically present as a clean distinction where all local-subnet communication works while all off-subnet communication fails. The symptoms point more directly to a routing issue than a layer 1 or layer 2 negotiation issue.
- C. Incorrect.
This is incorrect. SMB service availability on the file server is unrelated to the inability to reach external IP addresses. The successful ping to the file server shows local IP connectivity is working. Even if SMB were down, that would not explain why internet-bound traffic to 8.8.8.8 fails.
- D. Incorrect.
This is incorrect. With an address of 192.168.10.55 and a mask of 255.255.255.0, the PC is correctly configured for a typical /24 subnet containing the local server at 192.168.10.20. Changing the mask to 255.255.0.0 would expand the local network definition unnecessarily and would not resolve internet access if the gateway is absent or wrong. This distractor reflects a common misconception that larger subnets somehow improve connectivity.