Subnet Utilization Calculator
Compare recorded allocations with available capacity.
Subnet Utilization Calculator: inputs and results
Enter an IP address, CIDR prefix, or range. Then check its first and last address below.
Calculated Boundaries & ParametersReady
Key Capabilities
- •100% Client-Side: Zero telemetry or server latency.
- •RFC Compliant: Exact 32-bit and 128-bit integer precision.
- •Quick Copy: Instant access to CIDR, mask, and range bounds.
- •Cross-Platform: Works on desktop, tablet, and mobile browsers.
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How Subnet Utilization Calculator Works?
This tool works by dividing the number of recorded allocations in each subnet by its usable capacity and flagging subnets above set thresholds.
Enter the subnets
Paste each subnet CIDR.
Enter the used counts
Add the number of addresses in use, from IPAM, DHCP leases, or ARP tables.
Compute utilization
Used ÷ usable hosts × 100 %.
Apply thresholds
Subnets at 80 % or more are marked Warning, and subnets at 95 % or more are marked Critical.
Subnet Utilization Calculator Formula
The formula is Utilization = Used ÷ (2^(32 − p) − 2) × 100 %.
Utilization
U = Used ÷ Usable × 100 %Per subnet.
Free addresses
Usable − UsedRemaining capacity.
Aggregate utilization
Σ Used ÷ Σ Usable × 100 %Across all subnets.
Status
U ≥ 95 % Critical; U ≥ 80 % WarningCommon thresholds.
Worked example: 10.40.2.0/24 with 231 used
Usable = 254 Utilization = 231 ÷ 254 = 90.9 % Free = 23 Status = Warning
Subnet Utilization Calculator Conversion Table
The table rates 8 subnets by utilization with OK, Warning, and Critical thresholds.
| Subnet | Usable | Used | Free | Utilization | Status |
|---|---|---|---|---|---|
| 10.40.0.0/24 | 254 | 120 | 134 | 47.2 % | OK |
| 10.40.1.0/24 | 254 | 250 | 4 | 98.4 % | Critical |
| 10.40.2.0/24 | 254 | 231 | 23 | 90.9 % | Warning |
| 10.40.4.0/22 | 1,022 | 600 | 422 | 58.7 % | OK |
| 10.40.8.0/25 | 126 | 126 | 0 | 100.0 % | Critical |
| 10.40.8.128/26 | 62 | 30 | 32 | 48.4 % | OK |
| 10.40.9.0/27 | 30 | 27 | 3 | 90.0 % | Warning |
| 10.40.12.0/22 | 1,022 | 980 | 42 | 95.9 % | Critical |
What is Subnet Utilization Calculator?
The Subnet Utilization Calculator is a tool that measures how full each subnet is by comparing recorded allocations with usable capacity.
Utilization is the main health metric of an address plan. High utilization leads to DHCP exhaustion and failed device onboarding. Very low utilization in large subnets points to space that can be reclaimed.
The calculator works from recorded counts, so the result is only as accurate as the source. DHCP lease tables, ARP caches, and IPAM records each capture a slightly different picture.
What are the uses of Subnet Utilization Calculator?
There are 4 main uses of the Subnet Utilization Calculator, including capacity reviews, reclamation, and alerting.
Running capacity reviews
Monthly reviews list every subnet above the Warning threshold.
Reclaiming unused space
Subnets below 10 % utilization are candidates for shrinking.
Setting monitoring alerts
Thresholds from the calculator feed IPAM and monitoring alerts.
Reporting to management
Aggregate utilization summarizes the health of the whole estate.