Subnetting Quick Sheet
Subnetting
Overview
The IPv4 prefix table relates CIDR to mask, total addresses and usable hosts. Each additional prefix bit halves a block. Wildcard masks are the inverse of subnet masks. Remember special /31 and /32 behavior and that a mask must be contiguous.
Protocol stages
- 1Prefix
- 2Mask
- 3Block size
- 4Host capacity
| CIDR | Subnet mask | Addresses | Usable hosts |
|---|---|---|---|
| /21 | 255.255.248.0 | 2048 | 2046 |
| /22 | 255.255.252.0 | 1024 | 1022 |
| /23 | 255.255.254.0 | 512 | 510 |
| /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 |
/31: point-to-point, both addresses usable. /32: one address, no broadcast subnet.
Knowledge check
Moving from /24 to /25 creates what change?
Educational model — not real hardware, traffic or official certification.