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IP Address Capacity Planner

Plan an IP allocation: addresses needed today and after growth, the CIDR prefix that fits and the waste it leaves.

About IP Address Capacity Planner

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

Worked example

With the default values below, IP Address Capacity Planner returns Prefix Length per Subnet of /23. Change any input above and every figure here is recalculated in your browser.

Inputs used

Hosts to address todayaccepts 1–1,000,000 hosts
180 hosts
Expected growth over the planaccepts 0–500 %
40 %
Infrastructure addresses reservedaccepts 0–1,000 addresses
6 addresses
Subnets of this size requiredaccepts 1–100,000 subnets
12 subnets

Results

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

  1. 1Values used
  2. 2IP Address Capacity
  3. 3Prefix Length per Subnet= /23
  4. 4Addresses Needed per Subnet= 260
  5. 5Addresses in the Chosen Block= 512
  6. 6Address Space Wasted= 49.2
  7. 7Summary Prefix for All Subnets= /19

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, 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 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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IP Address Capacity Planner | Calcrivo