Lesson 1 of 3
Subnetting without a calculator
1 min
Subnetting looks like maths, but in practice you only need one table and one habit.
The table
| Prefix | Addresses | Usable hosts | Mask |
|---|---|---|---|
| /24 | 256 | 254 | 255.255.255.0 |
| /25 | 128 | 126 | 255.255.255.128 |
| /26 | 64 | 62 | 255.255.255.192 |
| /27 | 32 | 30 | 255.255.255.224 |
| /28 | 16 | 14 | 255.255.255.240 |
| /29 | 8 | 6 | 255.255.255.248 |
| /30 | 4 | 2 | 255.255.255.252 |
Each step down halves the block. Usable hosts are always the total minus 2 (network and broadcast addresses).
The habit: count in block sizes
A /27 is 32 addresses, so /27 networks start at multiples of 32: .0, .32, .64, .96…
Example: which /27 contains 192.168.10.77? 77 falls between 64 and 96, so:
- Network:
192.168.10.64 - First host:
.65, last host:.94 - Broadcast:
.95
Practise with three random addresses a day for a week. After that, you'll do it in your head.