IP Address Capacity Planner
Plan an IP allocation: addresses needed today and after growth, the CIDR prefix that fits and the waste it leaves.
Inputs
Prefix Length per Subnet
/23
Addresses Needed per Subnet
260
Addresses in the Chosen Block
512
Address Space Wasted
49.2%
Summary Prefix for All Subnets
/19
Step by step
Values used
Hosts to address today = 180 hosts; Expected growth over the plan = 40 %; Infrastructure addresses reserved = 6 addresses; Subnets of this size required = 12 subnets
IP Address Capacity
addresses = hosts × (1 + growth) + reservations + 2; prefix length = 32 − ceil(log2(addresses)).
Prefix Length per Subnet
= /23
Addresses Needed per Subnet
= 260
Addresses in the Chosen Block
= 512
Address Space Wasted
= 49.2
Summary Prefix for All Subnets
= /19
How it works
Because subnets only come in powers of two, the honest requirement is rounded up to the next binary boundary and everything between the two figures is waste. Aggregating the individual subnets needs a further log2 of the subnet count in shorter prefix bits. Right-sizing on the first pass avoids the two classic failures: a /24 for eight routers, and a subnet that runs out three months after go-live and forces a renumber.
Formula
IP Address Capacity
addresses = hosts × (1 + growth) + reservations + 2; prefix length = 32 − ceil(log2(addresses)).
- +2
- Network and broadcast addresses
- summary prefix
- Prefix that aggregates all the subnets into one advertisement
Frequently Asked Questions
How is IP Address Capacity calculated?
addresses = hosts × (1 + growth) + reservations + 2; prefix length = 32 − ceil(log2(addresses)). Because subnets only come in powers of two, the honest requirement is rounded up to the next binary boundary and everything between the two figures is waste. Aggregating the individual subnets needs a further log2 of the subnet count in shorter prefix bits.
Why does IP Address Capacity matter?
Right-sizing on the first pass avoids the two classic failures: a /24 for eight routers, and a subnet that runs out three months after go-live and forces a renumber.
What values do I need to enter?
This calculator takes 4 inputs: Hosts to address today, Expected growth over the plan, Infrastructure addresses reserved, Subnets of this size required. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
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