Quiz Entry - updated: 2026.07.14
What are the key subnetting formulas for calculating subnets and hosts?
Subnets = 2^n (n = bits borrowed); usable hosts = 2^h − 2 (h = host bits left), subtracting the network and broadcast addresses.
* The core trade-off: more subnets means fewer usable hosts per subnet. *
Subnetting Formulas:
| Formula | Purpose |
|---|---|
| 2^n | Number of subnets (n = borrowed bits) |
| 2^n - 2 | Number of usable hosts (n = remaining host bits) |
Why subtract 2 for hosts?
- Network address (all 0s in host portion) - cannot be assigned
- Broadcast address (all 1s in host portion) - cannot be assigned
Example: Subnetting 192.168.1.0/24 to /26
- Borrowed bits: 26 - 24 = 2 bits
- Subnets: 2^2 = 4 subnets
- Host bits remaining: 32 - 26 = 6 bits
- Hosts per subnet: 2^6 - 2 = 62 usable hosts
The 4 resulting subnets:
| Subnet | Network | First Host | Last Host | Broadcast |
|---|---|---|---|---|
| 1 | 192.168.1.0 | .1 | .62 | .63 |
| 2 | 192.168.1.64 | .65 | .126 | .127 |
| 3 | 192.168.1.128 | .129 | .190 | .191 |
| 4 | 192.168.1.192 | .193 | .254 | .255 |
Go deeper:
Subnetting Mastery (PracticalNetworking) — derives the 2^n / 2^h−2 formulas and the magic-number shortcut.
Subnet (Wikipedia) — the bit arithmetic behind subnet and host counts.