N10-009 Question 314
Single answerAddress pool exhaustion, Incorrect default gateway, Incorrect IP address, Duplicate IP address, Incorrect subnet maskA network technician is troubleshooting connectivity issues for users in a small office. All PCs should receive IPv4 settings automatically from DHCP on the 192.168.50.0/24 network. Several users report they can access local file servers on 192.168.50.10, but they cannot reach internet sites or a cloud-hosted CRM application. One affected PC has the following settings from ipconfig: IP address 192.168.50.87, subnet mask 255.255.255.0, default gateway 192.168.5.1, DHCP enabled Yes. Other affected PCs show different valid 192.168.50.x addresses with the same gateway. Which of the following is the MOST likely cause of the problem?
- A
Address pool exhaustion on the DHCP scope
- B
Incorrect default gateway handed out by DHCP
- C
Duplicate IP address on the network
- D
Incorrect subnet mask on the client
Show answer and explanation
Correct answer: B
Explanation
The best answer is the incorrect default gateway handed out by DHCP. In a /24 network such as 192.168.50.0/24, clients need a default gateway in the same subnet to forward traffic destined for remote networks. A gateway of 192.168.5.1 is not locally reachable from 192.168.50.87/24, so traffic to the internet or cloud services cannot leave the subnet. This aligns with the symptom pattern: local communication works, but off-subnet communication fails. From a troubleshooting perspective, this indicates a DHCP scope option misconfiguration, specifically Option 003 (Router/default gateway) in common DHCP implementations. Best practice is to verify DHCP scope options, confirm the configured router address matches the VLAN/subnet, and renew client leases after correcting the scope.
- A. Incorrect.
This is incorrect. If the DHCP address pool were exhausted, affected clients would typically fail to obtain a valid lease and might self-assign an APIPA address in the 169.254.0.0/16 range or show no usable IPv4 configuration. In this scenario, clients are receiving valid 192.168.50.x addresses and can communicate with local hosts, so DHCP is still leasing addresses.
- B. Correct.
This is correct. The clients are on the 192.168.50.0/24 network, so the default gateway should also be an address in that same subnet, such as 192.168.50.1. A gateway of 192.168.5.1 is outside the local subnet and cannot be reached directly without routing, which prevents access to remote networks like the internet or cloud applications. The fact that local resources remain reachable strongly points to a bad gateway configuration being distributed to multiple clients by DHCP.
- C. Incorrect.
This is incorrect. A duplicate IP address usually causes intermittent or severe connectivity issues for one or more hosts sharing the same address, often including ARP conflicts, inconsistent access to local resources, or duplicate IP warnings. Here, multiple clients have different valid IP addresses but the same incorrect gateway, making a duplicate address much less likely.
- D. Incorrect.
This is incorrect. An incorrect subnet mask can cause a host to misidentify which devices are local versus remote, often leading to broader communication failures. However, the reported subnet mask of 255.255.255.0 matches the intended 192.168.50.0/24 network. Since clients can consistently reach local servers in the same subnet, the mask is functioning as expected.