Size a Subnet for a Small Office or VPC

Quick prefix table

Devices (with headroom)	Prefix	Usable hosts
up to 14	/28	14
up to 30	/27	30
up to 62	/26	62
up to 126	/25	126
up to 254	/24	254

Steps

1. Count devices and double it for headroom, then pick the prefix from the table.
2. Open the Subnet Calculator with a private base address (e.g. 192.168.10.0) and your prefix.
3. Copy the network address, subnet mask, and usable host range into your router’s DHCP scope or cloud VPC config.
4. Reserve the first few host addresses for gateways and static gear; start the DHCP pool after them.
5. Adding a second subnet later (guest Wi-Fi, VoIP)? Re-run the calculator on the next block — e.g. 192.168.11.0/27 — and confirm the ranges do not overlap.
6. Validate any manually typed address with the IP Address Validator before saving.

Watch out

• Cloud VPCs often reserve extra addresses per subnet (AWS reserves 5), so a /28 yields fewer usable IPs than on-prem — size one step larger in the cloud.
• The wildcard mask shown by the calculator is what ACL rules expect — it is the inverse of the subnet mask, not the same thing.

Related

• Web Config Ops hub
• Subnet Calculator