N10-009 Question 315
Single answerAddress pool exhaustion, Incorrect default gateway, Incorrect IP address, Duplicate IP address, Incorrect subnet maskA network administrator receives several reports after deploying a new DHCP scope for the 192.168.50.0/24 office subnet. Some users can access internal servers and the internet normally. However, one user in Accounting cannot reach the internet or any devices outside the local subnet, even though they can successfully ping nearby PCs on the same floor. On the affected workstation, ipconfig shows the following: IP address 192.168.50.87, subnet mask 255.255.255.0, default gateway 192.168.50.254. The DHCP scope options were intended to assign a default gateway of 192.168.50.1, which is the router interface for that VLAN. Other users on the same subnet received 192.168.50.1 as their gateway and are working normally. Which of the following is the MOST likely cause of this user's connectivity issue?
- A
Address pool exhaustion on the DHCP server
- B
Incorrect default gateway on the affected host
- C
Duplicate IP address on the subnet
- D
Incorrect subnet mask assigned to the workstation
Show answer and explanation
Correct answer: B
Explanation
The best answer is the incorrect default gateway. A host uses its subnet mask to determine whether a destination is local or remote. For remote destinations, it forwards traffic to its configured default gateway. Since this workstation has a valid local IP configuration except for the gateway, it can communicate within 192.168.50.0/24 but cannot reach other subnets or the internet. This aligns with standard TCP/IP behavior documented in common networking references and vendor documentation such as Microsoft TCP/IP configuration guidance and Cisco routing fundamentals. In troubleshooting, if a client can reach local peers but not remote networks, verifying the default gateway is a best practice before investigating issues such as DHCP pool exhaustion, duplicate IPs, or subnet mask errors.
- A. Incorrect.
This is incorrect. DHCP address pool exhaustion typically prevents clients from obtaining a valid lease, often resulting in an APIPA address such as 169.254.x.x or failure to renew an address. In this scenario, the host has a valid IP address in the correct subnet and can communicate with local devices, so lease exhaustion is not the most likely issue.
- B. Correct.
This is correct. The workstation has a valid IP address and subnet mask for the 192.168.50.0/24 network, and it can reach local devices, which confirms local subnet communication is functioning. However, its default gateway is set to 192.168.50.254 instead of the intended router at 192.168.50.1. An incorrect default gateway would prevent traffic destined for remote networks, including the internet, from being forwarded properly.
- C. Incorrect.
This is incorrect. A duplicate IP address often causes intermittent connectivity, ARP conflicts, or messages about IP conflicts. It can disrupt both local and remote communication unpredictably. Here, the symptom pattern is consistent and specific: local subnet access works, but off-subnet access does not. That points more directly to a gateway problem than to an address conflict.
- D. Incorrect.
This is incorrect. If the subnet mask were wrong, the workstation might misidentify which hosts are local versus remote, causing broader communication issues. In this case, the subnet mask is 255.255.255.0, which is appropriate for a /24 subnet using addresses in 192.168.50.0-192.168.50.255. The host's ability to communicate with nearby devices on the same subnet further supports that the mask is correct.