Usable IP Calculator
Count how many addresses in a subnet you can actually assign after network, broadcast and infrastructure reservations.
Inputs
Assignable Addresses
246
Usable Hosts in Subnet
254
Total Reserved
8
Address Efficiency
96.1%
Step by step
Values used
Prefix length (CIDR) = 24 bits; Gateway / VRRP addresses reserved = 3 addresses; Other infrastructure reservations = 5 addresses
Usable IP
assignable = 2^(32 − p) − 2 − gateways − infrastructure reservations.
Assignable Addresses
= 246
Usable Hosts in Subnet
= 254
Total Reserved
= 8
Address Efficiency
= 96.1
How it works
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
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.