Starlight Tools
IP Subnet Utilization Calculator and Address Tracker
Calculate IPv4 subnet capacity, usable hosts, broadcast address, host range, available IP addresses, and used host percentage from CIDR, subnet mask, wildcard mask, IP range, or required hosts. Keep the tracker view for assigned, reserved, excluded, and unavailable addresses before a VLAN expansion, DHCP scope change, cloud subnet allocation, or IPAM cleanup.
Inputs
Choose a preset to see how provider-reserved addresses, DHCP exclusions, gateway pairs, and growth buffers should be counted.
Results
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Show subnet math
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Bulk subnet utilization report
Paste CSV rows with columns such as CIDR,VLAN,Site,VRF,Used,Reserved,Unavailable,Threshold. The report sorts by any column and classifies subnets as healthy, warning, exhausted, or underused.
Forecasting and IPv6 planning
Growth forecasting
IPv6 prefix allocation mode
IPv4 CIDR quick reference
Common subnet sizes from /16 through /32 with masks, wildcard masks, total addresses, usable hosts, and typical planning use cases.
| CIDR | Subnet mask | Wildcard mask | Total addresses | Usable hosts | Common use case |
|---|
How to use this IP utilization tracker
- Choose an input mode: enter CIDR, IP plus subnet mask, wildcard mask, IP range, or required hosts.
- Count used addresses: include active leases, static assignments, cloud interfaces, load balancer addresses, and IPAM records that are truly assigned.
- Add reserved and unavailable addresses: include gateways, VIPs, exclusions, quarantine holds, conflicts, and addresses intentionally kept out of service.
- Set a threshold: use your operational alert ceiling, often 70% to 85% for shared or high-churn networks.
- Review the result: compare available hosts, threshold headroom, growth buffer, and recommended prefix before changing address plans.
Formula and assumptions
Total addresses: 2^(32 - prefix)
Usable hosts: total addresses - 2 for traditional IPv4 subnets from /0 through /30.
Small subnet option: when enabled, /31 = 2 usable for point-to-point links and /32 = 1 usable for host routes.
Total consumed: used hosts + reserved hosts + other unavailable hosts
Available hosts: usable hosts - total consumed
Total utilization: total consumed / usable hosts
Recommended prefix: the smallest IPv4 prefix that can hold ceil((consumed + growth buffer) / alert threshold) usable hosts under the selected host-counting convention.
IPv4 mode models address counts. IPv6 mode models prefix allocation inside a parent block. This page does not discover hosts, scan networks, or validate ownership.
Example utilization scenarios
| Scenario | What to enter as used | What to enter as reserved or unavailable |
|---|---|---|
| Office VLAN | Active endpoints, printers, phones, and static host records | Gateway, HSRP/VRRP addresses, DHCP exclusions, future static ranges |
| Guest Wi-Fi subnet | Peak active leases or recent high-water DHCP count | Infrastructure addresses, excluded ranges, quarantine or captive portal holds |
| Cloud VPC subnet | Network interfaces, load balancers, NAT gateways, private endpoints | Provider-reserved addresses, planned expansion blocks, addresses kept for failover |
| Point-to-point /31 | Both link endpoint addresses | Usually none, if your network policy allows /31 point-to-point addressing |
Used, reserved, and available addresses mean different things
A subnet can look healthy if you only count active hosts, but become risky after static reservations, DHCP exclusions, provider-reserved addresses, and operational holds are included. This tracker treats those as consumed capacity because they cannot be handed to a new workload without a deliberate change.
The growth buffer is separate from current utilization. It estimates how many additional host addresses you want to leave room for, then checks whether the subnet can absorb that buffer while staying under your alert threshold.
Methodology
The calculator converts CIDR, subnet-mask, wildcard-mask, IP-range, or required-host inputs into a normalized IPv4 subnet, calculates total addresses, usable host capacity, broadcast address, and usable range, then subtracts used, reserved, and unavailable host counts. Utilization is calculated against usable hosts, not total addresses. The recommended prefix is the smallest prefix from /32 through /0 that can keep consumed plus growth-buffer demand under the selected utilization threshold.
Last reviewed: June 2026. Calculations are client-side arithmetic and do not inspect your network.
FAQs
How do I calculate IP utilization percentage?
Add used, reserved, excluded, and otherwise unavailable host addresses, divide that total by usable host capacity, then multiply by 100. For example, 185 consumed addresses in a /24 with 254 usable hosts is about 72.8% utilized.
How many usable IPs are in a /24?
A traditional IPv4 /24 has 256 total addresses and 254 usable host addresses because the network and broadcast addresses are not assigned to normal hosts.
What is a safe IP utilization threshold?
Many teams use 70% to 85%. Choose a lower threshold for guest Wi-Fi, labs, Kubernetes, cloud automation, or DHCP scopes with bursty demand.
Should DHCP exclusions count as used?
Count DHCP exclusions as reserved or unavailable for capacity planning. They are not available for normal dynamic assignment unless you change the DHCP scope policy.
Should reserved addresses count as utilized?
Yes for capacity planning. Even if a reserved address is not actively passing traffic, it is not freely available for normal assignment without changing the reservation or policy.
What is the difference between used, reserved, excluded, and available IPs?
Used IPs are assigned now. Reserved IPs are intentionally held for gateways, VIPs, static devices, provider reservations, or future allocations. Excluded IPs are kept out of automatic assignment. Available IPs are the usable hosts left after those categories are subtracted.
Why does the calculator normalize my CIDR block?
If you enter 10.20.30.17/24, that address belongs to the 10.20.30.0/24 network. The tracker shows the normalized network so results match the actual CIDR boundary.
Why do /31 and /32 networks behave differently?
Modern point-to-point /31 links can use both addresses, and /32 host routes can represent one address. Traditional LAN host counting treats network and broadcast addresses as unavailable, so the checkbox lets you choose the convention.
Can this scan my network?
No. This is a private planning calculator. It does not scan networks, query DHCP, inspect DNS, or connect to IPAM systems.
How do I calculate utilization from DHCP leases?
Use active leases or recent peak leases as used hosts. Then add reservations, exclusions, gateways, static assignments, cloud provider reservations, and addresses held for failover before checking utilization.
Does this support IPv6 utilization planning?
Yes, with a separate IPv6 prefix allocation mode. It tracks allocated, delegated, reserved, and available child prefixes inside a parent IPv6 block instead of pretending that IPv6 host exhaustion is the normal planning constraint.
Is this tracker private?
Yes. Inputs are processed locally and are not submitted to a backend.
Disclaimer
This is an infrastructure planning aid. Confirm address ownership, DHCP pools, DNS records, route tables, firewall policy, cloud provider reservations, and IPAM source-of-truth data before making production changes.