Subnetting Cheat Sheet (/24 to /30)

Subnetting cheat sheet

| Prefix | Mask | Block size | Usable hosts | |---|---|---|---| | /24 | 255.255.255.0 | 256 | 254 | | /25 | 255.255.255.128 | 128 | 126 | | /26 | 255.255.255.192 | 64 | 62 | | /27 | 255.255.255.224 | 32 | 30 | | /28 | 255.255.255.240 | 16 | 14 | | /29 | 255.255.255.248 | 8 | 6 | | /30 | 255.255.255.252 | 4 | 2 |

Method: block size = 256 − last mask octet. Subnets start at multiples of the block size. For 192.168.1.77/27: block 32 → subnets .0, .32, .64, .96 → .77 is in .64; first host .65, last host .94, broadcast .95.

Wildcard mask (OSPF, ACLs) = 255.255.255.255 − subnet mask. /26 → 0.0.0.63.

Practise it for real in the CCNA: Subnetting in Practice lab.