Count how many addresses in a subnet you can actually assign after network, broadcast and 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. Capacity planning that ignores reservations is how a /24 sized for 250 hosts runs out at 240 and forces an emergency re-address.
Usable IP
assignable = 2^(32 − p) − 2 − gateways − infrastructure reservations.
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.
Capacity planning that ignores reservations is how a /24 sized for 250 hosts runs out at 240 and forces an emergency re-address.
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.