Subnetting Quick Reference — The 60-Second Method on One Page

Subnetting in 60 seconds. No tables to memorize. No binary conversion. One rule, three results.

If you came from the IG reel — scroll to the magic-number table. Save the page. The whole method fits on one screen.

The one rule

Subtract the interesting octet of the subnet mask from 256. The result is your block size.

The “interesting octet” is the one that’s not 0 and not 255. For 255.255.255.224, the interesting octet is 224. So:

256 - 224 = 32  ← block size

From the block size, three answers fall out:

1. Subnet boundaries — start at 0, add the block size repeatedly: 0, 32, 64, 96, 128, 160, 192, 224
2. Network address for any IP — find the boundary the IP falls into
3. Broadcast address — next boundary minus 1

That’s the whole method. Three numbers from one subtraction.

The magic-number table — memorize this

The only thing you need to memorize is which mask octet maps to which block size. There are 8 values that ever appear in subnet masks past 255.0.0.0:

Sixteen numbers total. Memorize them. They’re the entire arithmetic surface of every subnetting question on the exam.

Prefix-to-mask conversion (the other thing to memorize)

When the exam gives you /24 instead of 255.255.255.0, you need the conversion:

Memorize cold: /24, /27, /28, /29, /30 — those are the most common on the exam.

The /29 walkthrough — the most-tested subnet

Given: 10.0.1.0/29 — list the first four subnets.

Step 1: /29 = 255.255.255.248. Interesting octet: 248. Block size: 256 - 248 = 8.

Step 2: Subnets step by 8 in the fourth octet.

Five values per row, only the network column needs calculation. Everything else is +1 / -1.

The four traps to avoid under time pressure

1. Off-by-one on usable hosts. A /29 has 8 total addresses but 6 usable. The exam will ask “how many usable hosts” specifically to catch people who answer 8. Subtract 2 for network + broadcast — always, unless the question explicitly asks for total.

2. Wrong interesting octet. For /20, the interesting octet is the third octet (240), not the fourth. The interesting octet is whichever one isn’t 0 and isn’t 255. If all four octets are 0 or 255, the mask is on a classful boundary and there’s no math required.

3. Block size in the wrong octet. When the interesting octet is the third, subnets step in the third octet. The fourth octet ranges 0–255 within each /20 subnet. Don’t apply the block size to the wrong octet.

4. Math errors under stress. 256 − 224 is 32, not 30. 256 − 248 is 8, not 6. Practice the 8 specific subtractions (256 minus 128, 192, 224, 240, 248, 252, 254, 255) until they’re reflex.

VLSM in 60 seconds

The exam loves VLSM questions: “split 192.168.10.0/24 into subnets for 50, 25, 12, and 5 hosts.”

The rule: always allocate the largest first, work down.

• 50 hosts → needs 52 addresses → /26 (62 usable) → 192.168.10.0/26 (covers .0–.63)
• 25 hosts → needs 27 addresses → /27 (30 usable) → 192.168.10.64/27 (.64–.95)
• 12 hosts → needs 14 addresses → /28 (14 usable) → 192.168.10.96/28 (.96–.111)
• 5 hosts → needs 7 addresses → /29 (6 usable) → 192.168.10.112/29 (.112–.119)

Always largest-first. If you allocate small ones first, you fragment the space and the large allocations may not fit.

The deep version

For the underlying why — why 256 minus the mask works, what the binary actually does, and the full 5-question drill that builds the muscle memory — see the deep companion: Subnetting Without a Calculator — The Magic-Number Method.

Drill these problems for free

The subnet calculator on network.it-learn.io gives you unlimited practice problems with instant answers — type in your network, see if you got the right boundaries, repeat. 5 minutes a day for a week and the method is muscle memory.