Subnetting PBQ Lab 2: Wrong Subnet

Find the workstation that cannot communicate because its IP address belongs to a different subnet.

Scenario

Help Desk Ticket

The office LAN uses 192.168.20.0/27. All workstations should use the gateway 192.168.20.1. One workstation cannot reach shared folders or the gateway.

  1. Calculate the /27 block size.
  2. Identify the correct usable host range.
  3. Find the device outside the subnet.

Network Diagram

🌐

Default Gateway

Router Interface

IP Address: 192.168.20.1

Subnet Mask: 255.255.255.224 (/27)

🖥️

PC-A

IP Address192.168.20.5
Subnet Mask255.255.255.224
Gateway192.168.20.1
Reported: Working
🖥️

PC-B

IP Address192.168.20.18
Subnet Mask255.255.255.224
Gateway192.168.20.1
Reported: Working
🖥️

PC-C

IP Address192.168.20.33
Subnet Mask255.255.255.224
Gateway192.168.20.1
Reported: Not Working
🖥️

PC-D

IP Address192.168.20.29
Subnet Mask255.255.255.224
Gateway192.168.20.1
Reported: Working

Subnet Work Area

ItemStudent Work
CIDR/27
Subnet Mask255.255.255.224
Block Size256 - 224 = ______
Network Address192.168.20.______
Broadcast Address192.168.20.______
Usable Host Range192.168.20.______ through 192.168.20.______

Answer the PBQ

Instructor Answer Key

Bad device: PC-C

Why: 192.168.20.33/27 is in the 192.168.20.32/27 subnet, not the 192.168.20.0/27 subnet.

  • Network: 192.168.20.0
  • Broadcast: 192.168.20.31
  • Usable: 192.168.20.1–192.168.20.30
  • Correction: Change PC-C to a valid unused IP in that range.

Instructor Debrief Script

“The gateway tells us which subnet this LAN is using. With /27, the block size is 32. The first network is .0 through .31. PC-C is .33, so it has stepped into the next subnet.”

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