Subnetting and VLSM Design Lab

Why Subnetting PBQs Matter on N10-009

The CompTIA Network+ N10-009 exam contains a maximum of 90 questions in 90 minutes, a mix of multiple-choice and performance-based questions (PBQs). PBQs usually appear in the first few minutes of the exam, are weighted more heavily than single multiple-choice items, and consume time quickly. The passing score is 720 on a 100-900 scale. A common mistake is to burn fifteen minutes on the first subnetting PBQ - flag it, move on, and return after answering the faster multiple-choice items.

Subnetting PBQs typically ask you to complete a table, drag subnets into the correct slot, or choose the best mask for several departments. The safest approach is always the same: size each network, allocate the largest network first, 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 spans 10.40.8.0 through 10.40.11.255 - that is 1024 total addresses. A /22 borrows two host bits from a /24, so it covers four contiguous /24 blocks (8, 9, 10, 11).

Step 1: Choose Prefix Lengths

Usable hosts = 2^(host bits) - 2. The minus two accounts for the reserved network and broadcast addresses.

Note that 70 hosts will not fit in a /26 (62 usable), so it rounds up to a /25. This rounding catches many test takers.

Step 2: Allocate Largest to Smallest

Unused space remains from 10.40.10.56 through 10.40.11.255 - roughly 456 addresses for growth, another VLAN, loopbacks, or future WAN circuits.

PBQ Method (Repeatable Six Steps)

1. Write host requirements from largest to smallest.
2. Convert each requirement to the smallest prefix that fits (round up).
3. Start at the first address in the assigned block.
4. Move forward by the block size of each subnet (block size = 256 minus the mask value in the interesting octet).
5. Place the default gateway at the first usable address consistently.
6. Verify each broadcast address is exactly one less than the next network ID - no gaps, no overlap.

Fast Mask Reference

Memorize this table cold; it eliminates arithmetic under pressure.

A fast trick: the interesting octet is wherever the mask is neither 255 nor 0. The block size is 256 minus that octet value. For /27 (mask .224) the block size is 256 - 224 = 32, so subnets start at .0, .32, .64, .96, and so on.

Worked Mental Check

Given 10.40.10.32/28, which subnet does host 10.40.10.40 belong to? Block size for /28 is 16, so subnets begin at .0, .16, .32, .48. Address .40 falls in the .32 block; usable range .33 - .46; broadcast .47. The gateway .33 is valid; .47 would be the broadcast and must never be a gateway.

Common PBQ Traps

• Allocating small subnets first, leaving no aligned space for a larger network.
• Using the broadcast or network address as a host or gateway.
• Forgetting a /30 has only two usable addresses for a point-to-point link.
• Overlapping two subnets in the same range because block size was miscalculated.
• Choosing a prefix that meets today's count but ignores a stated growth requirement.
• Mixing CIDR notation with the wrong dotted-decimal mask (for example pairing /26 with 255.255.255.128).