While studying for my CCNA, I found myself constantly flipping back and forth to reference subnet masks and CIDR notations.
Eventually, I found this cheat table — and it’s now stuck on the wall above my desk. If you’re getting subnetting drilled into your head daily, this might help you too.
Subnetting Cheat Table
| Group Size (IP Host Range / Block Size) | 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 |
|---|---|---|---|---|---|---|---|---|
| Subnet Mask | 128 | 192 | 224 | 240 | 248 | 252 | 254 | 255 |
| CIDR Notation /4th Octet | /25 | /26 | /27 | /28 | /29 | /30 | /31 | /32 |
| CIDR Notation /3rd Octet | /17 | /18 | /19 | /20 | /21 | /22 | /23 | /24 |
| CIDR Notation /2nd Octet | /9 | /10 | /11 | /12 | /13 | /14 | /15 | /16 |
| CIDR Notation /1st Octet | /1 | /2 | /3 | /4 | /5 | /6 | /7 | /8 |
How I Use It
- Quickly identify subnet mask values for different CIDR ranges
- Understand block sizes per octet
- Speed up subnetting questions in labs and exams
💡 Stick this table in your notes or keep it open during practice — you’ll thank yourself later.
More subnetting deep dives to come. Happy hacking. -D3