Subnetting PBQ Lab 3: Wrong Subnet Mask

Find the workstation that has the correct IP address but the wrong subnet mask.

Scenario

Help Desk Ticket

The network was recently divided into smaller /26 subnets. Four computers are connected to the same VLAN. One computer has the right IP range but the wrong subnet mask.

Network Diagram

🌐

Default Gateway

Router Interface

IP Address: 10.10.5.65

Subnet Mask: 255.255.255.192 (/26)

🖥️

PC-A

IP Address10.10.5.70
Subnet Mask255.255.255.192
Gateway10.10.5.65
Reported: Working
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PC-B

IP Address10.10.5.88
Subnet Mask255.255.255.0
Gateway10.10.5.65
Reported: Intermittent/Incorrect
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PC-C

IP Address10.10.5.100
Subnet Mask255.255.255.192
Gateway10.10.5.65
Reported: Working
🖥️

PC-D

IP Address10.10.5.120
Subnet Mask255.255.255.192
Gateway10.10.5.65
Reported: Working

Subnet Work Area

ItemStudent Work
Required CIDR/26
Required Mask255.255.255.______
Block Size256 - 192 = ______
Network Address10.10.5.______
Broadcast Address10.10.5.______
Usable Host Range10.10.5.______ through 10.10.5.______

Answer the PBQ

Instructor Answer Key

Bad device: PC-B

Why: PC-B uses 255.255.255.0 instead of 255.255.255.192. Its IP is valid, but its mask is incorrect.

  • Correct subnet: 10.10.5.64/26
  • Broadcast: 10.10.5.127
  • Usable: 10.10.5.65–10.10.5.126
  • Correction: Set PC-B mask to 255.255.255.192

Instructor Debrief Script

“This one is powerful because the IP address is not the problem. The mask changes how the computer sees the network.”

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