Network+ Style PBQ: Subnetting Troubleshooting Lab

Analyze IP addressing, subnet masks, gateway placement, subnet boundaries, and host usability to identify the workstation that cannot communicate.

Scenario

Help Desk Ticket

A small office uses one LAN subnet. Four computers connect to the same switch and should communicate with the default gateway. One computer cannot reach the gateway or other devices.

  1. Review each computer's IP address, subnet mask, and gateway.
  2. Calculate the correct subnet range.
  3. Find the bad workstation.
  4. Select the best correction.

Network Diagram

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Default Gateway

Router G0/0

IP Address: 192.168.10.65

Subnet Mask: 255.255.255.192 (/26)

🖥️

PC-A

IP Address192.168.10.66
Subnet Mask255.255.255.192
Gateway192.168.10.65
Reported: Working
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PC-B

IP Address192.168.10.94
Subnet Mask255.255.255.192
Gateway192.168.10.65
Reported: Working
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PC-C

IP Address192.168.10.127
Subnet Mask255.255.255.192
Gateway192.168.10.65
Reported: Not Working
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PC-D

IP Address192.168.10.100
Subnet Mask255.255.255.192
Gateway192.168.10.65
Reported: Working

Subnet Work Area

ItemStudent Work
Given CIDR/26
Subnet Mask255.255.255.192
Block Size256 - 192 = ______
Subnet Ranges0–63, 64–127, 128–191, 192–255
Network Address192.168.10.______
Broadcast Address192.168.10.______
Usable Host Range192.168.10.______ through 192.168.10.______

Answer the PBQ

Instructor Answer Key

Bad computer: PC-C

Reason: PC-C is configured with 192.168.10.127/26. In the 192.168.10.64/26 subnet, .127 is the broadcast address and cannot be assigned to a host.

Subnet Breakdown

  • Subnet mask: 255.255.255.192
  • CIDR: /26
  • Block size: 256 - 192 = 64
  • Subnet containing gateway: 192.168.10.64/26
  • Network address: 192.168.10.64
  • First usable: 192.168.10.65
  • Last usable: 192.168.10.126
  • Broadcast: 192.168.10.127

Fix: Assign PC-C a valid unused host address from 192.168.10.65 through 192.168.10.126.

Instructor Debrief Script

“This is where subnetting becomes troubleshooting. The numbers tell us who belongs on the same network and which addresses are usable. With a /26, the block size is 64. Since the gateway is 192.168.10.65, the subnet starts at 192.168.10.64. The broadcast address is 192.168.10.127. That means usable hosts are 192.168.10.65 through 192.168.10.126. PC-C is using the broadcast address, so it cannot communicate correctly.”

Variation for extra practice: Change PC-C to 192.168.10.130 /26. Then the failure is caused by PC-C being in a different subnet: 192.168.10.128–191.

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