220-1101 Question 150
Single answerSubnet maskA technician is troubleshooting a desktop that cannot communicate with other devices on the local office network. The PC has the following IPv4 settings: IP address 192.168.10.45, subnet mask 255.255.255.0, and default gateway 192.168.20.1. Other users on the same switch are using addresses in the 192.168.20.x range with a subnet mask of 255.255.255.0 and can access shared printers and file servers. Which change should the technician make first to restore local network connectivity?
- A
Change the PC's subnet mask to 255.255.0.0
- B
Change the PC's default gateway to 192.168.10.1
- C
Change the PC's IP address to 192.168.20.45
- D
Change the PC's DNS server to match the file server's IP address
Show answer and explanation
Correct answer: C
Explanation
A subnet mask determines which portion of an IPv4 address identifies the network and which portion identifies the host. With a subnet mask of 255.255.255.0, the first three octets define the network. That means 192.168.10.45 is on network 192.168.10.0/24, while devices using 192.168.20.x are on network 192.168.20.0/24. Hosts on different subnets require routing to communicate, and in a standard office LAN where all nearby devices are intended to be on the same local subnet, the first step is to correct the host's IP address so it matches the local network. This aligns with common TCP/IP troubleshooting practices documented by major networking vendors and operating system documentation: verify IP address, subnet mask, and default gateway belong to the intended network before troubleshooting higher-layer settings such as DNS.
- A. Incorrect.
Incorrect. Changing the subnet mask to 255.255.0.0 would make the host treat 192.168.10.x and 192.168.20.x as part of the same larger network, but that would not match the rest of the office configuration shown in the scenario. In a typical small office using 255.255.255.0, widening the mask on one PC creates an inconsistent configuration and can lead to routing and communication issues. This reflects a common misconception that subnet masks should be changed to make mismatched addresses work.
- B. Incorrect.
Incorrect. Changing the default gateway to 192.168.10.1 would place the gateway in the same subnet as the PC's current address, but the scenario indicates the office network is using the 192.168.20.0/24 subnet. The main issue is that this PC is addressed for a different subnet than the rest of the local devices. The gateway setting matters for reaching remote networks, but it does not solve the fact that the host is on the wrong local network for this environment.
- C. Correct.
Correct. With a subnet mask of 255.255.255.0, the address 192.168.10.45 belongs to the 192.168.10.0/24 subnet, while the other office devices are on 192.168.20.0/24. Because the PC is configured for a different subnet, it will not communicate locally as expected with devices in 192.168.20.x without routing. Changing the IP address to 192.168.20.45 places the PC in the same subnet as the rest of the office network, which is the first correction needed.
- D. Incorrect.
Incorrect. DNS is used for name resolution, not for basic layer 3 subnet membership. Even with incorrect DNS settings, devices on the same subnet can usually still communicate by IP address. Since the problem described is local network connectivity and the IP configuration shows the host is in the wrong subnet, changing DNS would not address the root cause.