Plan before calculating
Start with device counts, gateways, infrastructure addresses, DHCP behaviour, growth and routing boundaries. A subnet that fits today may be wrong for a network expected to double.
CIDR and subnet size
CIDR expresses the prefix length, such as /24. For IPv4, total addresses are 2^(32-prefix). Traditional host planning reserves network and broadcast addresses.
| Prefix | Total | Traditional usable hosts |
|---|---|---|
| /24 | 256 | 254 |
| /25 | 128 | 126 |
| /26 | 64 | 62 |
| /27 | 32 | 30 |
| /28 | 16 | 14 |
VLSM
Variable Length Subnet Masking lets different networks use different prefix lengths. Large user networks can use larger prefixes while point-to-point infrastructure uses small ones.
DHCP and reservations
Decide which addresses are static, DHCP-reserved or dynamically leased. Gateways and critical infrastructure usually need predictable addresses. Document reservations alongside DNS and management records.
Growth and summarisation
Leave contiguous space for future networks where possible. This makes route summarisation easier and keeps the address plan maintainable.
Design checklist
- List devices and expected growth.
- Define Layer 2 and Layer 3 boundaries.
- Allocate gateway and infrastructure addresses.
- Size DHCP scopes with headroom.
- Check for overlaps.
- Document prefix, gateway, DHCP range and DNS together.
- Consider IPv6 alongside IPv4.
Related tools
Calculate and validate address ranges before deployment.