Usable IP Calculator
Count how many addresses in a subnet you can actually assign after network, broadcast and infrastructure reservations.
About Usable IP Calculator
Start from the theoretical host count, subtract the network and broadcast addresses, then subtract the addresses your design reserves for gateways, load-balancer VIPs and management interfaces. Capacity planning that ignores reservations is how a /24 sized for 250 hosts runs out at 240 and forces an emergency re-address.
Formula
Usable IP
assignable = 2^(32 − p) − 2 − gateways − infrastructure reservations.
- p
- CIDR prefix length
- gateways
- Router/VRRP addresses held back
Worked example
With the default values below, Usable IP Calculator returns Assignable Addresses of 246. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Prefix length (CIDR)accepts 0–32 bits
- 24 bits
- Gateway / VRRP addresses reservedaccepts 0–16 addresses
- 3 addresses
- Other infrastructure reservationsaccepts 0–64 addresses
- 5 addresses
Results
- Assignable Addresses
- 246
- Usable Hosts in Subnet
- 254
- Total Reserved
- 8
- Address Efficiency
- 96.1%
Step by step
- 1Values used
- 2Usable IP
- 3Assignable Addresses= 246
- 4Usable Hosts in Subnet= 254
- 5Total Reserved= 8
- 6Address Efficiency= 96.1
Assumptions and limits
- The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, fees or charges or compounding frequency into account.
- Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Usable IP calculated?
assignable = 2^(32 − p) − 2 − gateways − infrastructure reservations. Start from the theoretical host count, subtract the network and broadcast addresses, then subtract the addresses your design reserves for gateways, load-balancer VIPs and management interfaces.
Why does Usable IP matter?
Capacity planning that ignores reservations is how a /24 sized for 250 hosts runs out at 240 and forces an emergency re-address.
What values do I need to enter?
This calculator takes 3 inputs: Prefix length (CIDR), Gateway / VRRP addresses reserved, Other infrastructure reservations. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.