CCNA Subnetting Reference
Subnetting under time pressure is the CCNA skill. Memorize this table and the block-size trick, and subnet questions become seconds, not minutes.
CIDR / mask / host reference
| CIDR | Subnet mask | Usable hosts | Block size |
|---|---|---|---|
| /24 | 255.255.255.0 | 254 | 256 |
| /25 | 255.255.255.128 | 126 | 128 |
| /26 | 255.255.255.192 | 62 | 64 |
| /27 | 255.255.255.224 | 30 | 32 |
| /28 | 255.255.255.240 | 14 | 16 |
| /29 | 255.255.255.248 | 6 | 8 |
| /30 | 255.255.255.252 | 2 | 4 |
| /23 | 255.255.254.0 | 510 | 512 (2nd octet) |
| /22 | 255.255.252.0 | 1,022 | 1,024 |
| /16 | 255.255.0.0 | 65,534 | , |
Fast method
| Step | Do this |
|---|---|
| 1 | Usable hosts = 2^(host bits) − 2 |
| 2 | Block size = 256 − (mask octet value) |
| 3 | Networks increment by block size in the 'interesting' octet |
| 4 | Network address = the block start ≤ the host's octet |
| 5 | Broadcast = next block start − 1 |
Private address ranges (RFC 1918)
| Range | CIDR | Class |
|---|---|---|
| 10.0.0.0 - 10.255.255.255 | 10.0.0.0/8 | A |
| 172.16.0.0 - 172.31.255.255 | 172.16.0.0/12 | B |
| 192.168.0.0 - 192.168.255.255 | 192.168.0.0/16 | C |
| 169.254.0.0 - 169.254.255.255 | 169.254.0.0/16 | APIPA |
Ready to test yourself?
Memorizing a table is a start, practice questions make it stick.