Subnetting trips up every networking beginner until it suddenly clicks. This cheat sheet gives you the tables and the method; practise it in the free CCNA course and check your answers with the subnet calculator.
CIDR to subnet mask
/24= 255.255.255.0 = 256 addresses (254 hosts)/25= 255.255.255.128 = 128 addresses (126 hosts)/26= 255.255.255.192 = 64 addresses (62 hosts)/27= 255.255.255.224 = 32 addresses (30 hosts)/28= 255.255.255.240 = 16 addresses (14 hosts)/29= 255.255.255.248 = 8 addresses (6 hosts)/30= 255.255.255.252 = 4 addresses (2 hosts)
The numbers you memorise
- Hosts per subnet = 2(32 − prefix) − 2 (minus network and broadcast)
- The mask octet values: 128, 192, 224, 240, 248, 252, 254, 255
- Block size = 256 − mask octet (e.g. 256 − 192 = 64)
How to subnet, step by step
- Find the block size from the mask (256 − interesting octet).
- List the networks by counting up in block-size steps (0, 64, 128, 192…).
- The network address is the start of the block.
- The broadcast is one below the next network.
- Usable hosts are everything in between.
Example: 192.168.1.0/26 → block size 64 → networks .0, .64, .128, .192. For .64: broadcast .127, hosts .65–.126.
Practise until it is automatic
Work real subnet problems in CCNA subnet math and subnet design, and use the subnet calculator to check yourself instantly. The full free CCNA course takes you from binary to routing.