Subnetting Made Simple: A Fast Method for the CCNA and Real Jobs
Subnetting scares many CCNA candidates, but it is just arithmetic with a few patterns. Once you know the block size method, most questions take under 30 seconds.
The only table you need to memorize
| Prefix (in the interesting octet) | Mask value | Block size |
|---|---|---|
| /25 or /17 or /9 | 128 | 128 |
| /26 or /18 or /10 | 192 | 64 |
| /27 or /19 or /11 | 224 | 32 |
| /28 or /20 or /12 | 240 | 16 |
| /29 or /21 or /13 | 248 | 8 |
| /30 or /22 or /14 | 252 | 4 |
| /31 or /23 or /15 | 254 | 2 |
| /32 or /24 or /16 | 255 | 1 |
Block size = 256 − mask value. The "interesting octet" is the one where the mask is not 255 or 0.
The method in four steps
- Find the interesting octet and its block size.
- List multiples of the block size in that octet: 0, 32, 64, 96...
- The network address is the multiple at or just below your IP's value.
- The broadcast address is one less than the next multiple. Usable hosts are everything in between.
Example 1: 192.168.10.77/27
- /27 → mask 255.255.255.224 → block size 32 in the fourth octet.
- Multiples: 0, 32, 64, 96. 77 falls between 64 and 96.
- Network: 192.168.10.64. Broadcast: 192.168.10.95.
- Usable: 192.168.10.65 – 192.168.10.94 (30 hosts, since 2^5 − 2 = 30).
Example 2: 172.16.45.200/20
- /20 → mask 255.255.240.0 → interesting octet is the third, block size 16.
- Multiples in the third octet: 0, 16, 32, 48. 45 falls between 32 and 48.
- Network: 172.16.32.0. Broadcast: 172.16.47.255.
- Usable hosts: 2^12 − 2 = 4,094.
Example 3: designing subnets
"You need at least 50 hosts per subnet from 10.1.1.0/24. What prefix?"
- 50 hosts needs 6 host bits (2^6 − 2 = 62). 32 − 6 = /26.
- That gives four subnets: .0, .64, .128, .192.
Hosts and subnets formulas
- Usable hosts = 2^(host bits) − 2 (except /31 point-to-point links and /32 host routes).
- Number of subnets = 2^(borrowed bits).
Practice problems
- 10.20.30.130/26 – network and broadcast?
- 192.168.1.9/29 – usable range?
- 172.31.100.5/22 – network address?
- Smallest prefix that fits 500 hosts?
Answers: 1) 10.20.30.128 / 10.20.30.191. 2) 192.168.1.9–192.168.1.14 (network .8, broadcast .15). 3) 172.31.100.0. 4) /23 (510 hosts).
Make it automatic
Do ten problems a day. Write random IPs on paper, solve them, and check with a subnet calculator afterward, never before. Within two weeks, subnetting goes from your weakest topic to free points. Pair this with our CCNA study plan.