CIDR Subnet Trainer & Calculator
Updated Mar 6, 2026
A fast subnet calculator for working engineers plus a built-in training resource for learning subnetting from scratch. Enter any IP with a CIDR prefix or subnet mask and instantly see the network, broadcast, host range, wildcard mask, and more.
Includes a quick-reference subnet table and a step-by-step guide to subnetting without a calculator — perfect for certification prep or daily network engineering.
Subnet Calculator
Enter an IP with CIDR prefix or subnet mask.
Subnet Reference Table
Common subnet sizes at a glance.
| CIDR | Subnet Mask | Usable Hosts | Block Size |
|---|---|---|---|
| /30 | 255.255.255.252 | 2 | 4 |
| /29 | 255.255.255.248 | 6 | 8 |
| /28 | 255.255.255.240 | 14 | 16 |
| /27 | 255.255.255.224 | 30 | 32 |
| /26 | 255.255.255.192 | 62 | 64 |
| /25 | 255.255.255.128 | 126 | 128 |
| /24 | 255.255.255.0 | 254 | 256 |
How to Subnet Without a Calculator
A quick, repeatable method you can do on paper or in your head.
Step 1 — Find the Block Size
Subtract the “interesting” octet of the subnet mask from 256. The interesting octet is the one that isn't 255 or 0.
Example: /26
Mask = 255.255.255.192
Interesting octet = 192
Block size = 256 − 192 = 64
Step 2 — List the Subnet Boundaries
Starting from 0, count up by the block size in the interesting octet. Each number is the start of a new subnet.
Block size = 64
Subnets start at:
.0 .64 .128 .192
Step 3 — Find Where Your IP Falls
Look at the host octet of your IP address and find which subnet range it belongs to. The range starts at one boundary and ends just before the next.
IP: 192.168.10.70
Boundaries: .0 .64 .128 .192
70 falls between 64 and 128
→ Subnet: 192.168.10.64/26
Step 4 — Determine Network & Broadcast
The network address is the subnet boundary your IP lands on. The broadcast address is one less than the next boundary.
Network = 192.168.10.64
Broadcast = 192.168.10.127 (128 − 1)
First host = 192.168.10.65
Last host = 192.168.10.126
Usable hosts = 64 − 2 = 62
Quick Cheat Sheet
| Prefix | Mask Octet | Block | Hosts |
|---|---|---|---|
| /25 | 128 | 128 | 126 |
| /26 | 192 | 64 | 62 |
| /27 | 224 | 32 | 30 |
| /28 | 240 | 16 | 14 |
| /29 | 248 | 8 | 6 |
| /30 | 252 | 4 | 2 |
Memorize this table and you can subnet any address in seconds. The block size is always 256 − mask_octet.