Subnetting and VLSM Design Lab

Performance-based subnetting questions often ask you to complete a table, drag subnets into the correct place, or choose the best mask for multiple departments. The safest approach is to size the networks first, allocate the largest networks first, and then verify that no ranges overlap.

Scenario

You are given the private block 10.40.8.0/22. Design subnets for a branch office:

The /22 gives addresses from 10.40.8.0 through 10.40.11.255. That is 1024 total addresses.

Step 1: Choose Prefix Lengths

Remember that IPv4 subnets reserve the network and broadcast addresses. A /25 has 128 total addresses and 126 usable host addresses.

Step 2: Allocate Largest to Smallest

Unused space remains from 10.40.10.56 through 10.40.11.255. That is useful for growth, another VLAN, loopbacks, or future WAN circuits.

PBQ Method

1. Write the host requirements from largest to smallest.
2. Convert each host requirement to the smallest prefix that fits.
3. Start at the first address in the assigned block.
4. Move forward by the subnet block size.
5. Use the first or last usable address as the default gateway consistently.
6. Verify that each broadcast address is one less than the next network ID.

Fast Mask Reference

Common PBQ Traps

• Allocating small subnets first and leaving no aligned space for a larger network.
• Using the broadcast address as a gateway.
• Forgetting that a /30 has only two usable IPv4 addresses.
• Overlapping two subnets in the same address range.
• Choosing a prefix that meets today's count but leaves no room when the prompt explicitly asks for growth.
• Mixing CIDR notation and dotted decimal masks incorrectly.