DHCP Scope
Plan a DHCP address pool scope and calculate available leases within a subnet.
About DHCP Scope
A DHCP scope's usable address count is the total addresses in the start-to-end range minus any addresses reserved for static assignments or explicitly excluded from the pool: usable = (end_ip − start_ip + 1) − reserved. Comparing currently active leases against that usable count gives the lease utilization percentage, a key capacity-planning metric — pools running consistently above 80% utilization risk running out of addresses during peak demand (e.g. a Monday morning influx of devices), which causes new devices to fail to obtain an IP address entirely.
Formulas
Usable DHCP addresses
usable = (end_ip − start_ip + 1) − reserved
- E
- End IP as a 32-bit integer
- S
- Start IP as a 32-bit integer
- r
- Reserved/excluded addresses
Lease utilization
utilization% = active_leases / usable_addresses × 100
Worked example
With the default values below, DHCP Scope returns Lease Pool Utilization of 64.94%. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Scope Start IP
- 192.168.1.10
- Scope End IP
- 192.168.1.250
- Reserved/Excluded Addressesminimum 0
- 10
- Currently Active Leasesminimum 0
- 150
Results
- Lease Pool Utilization
- 64.94%
- Usable Addresses in Scope
- 231
- Available for New Leases
- 81
- Total Addresses in Range
- 241
- Capacity Status
- Moderate — healthy utilization
Step by step
- 1Values used
- 2Usable DHCP addresses
- 3Lease utilization
- 4Lease Pool Utilization= 64.94
- 5Usable Addresses in Scope= 231
- 6Available for New Leases= 81
- 7Total Addresses in Range= 241
- 8Capacity Status= Moderate — healthy utilization
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, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, 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
Why subtract reserved addresses from the scope's total range?
Reserved addresses (static IP reservations for servers/printers, or explicit exclusions like a gateway or existing static assignment within the range) are set aside so DHCP will never hand them out dynamically, which means they don't count toward the pool available for dynamic lease assignment.
What lease utilization percentage should trigger action?
Many administrators treat 80% sustained utilization as a warning threshold to start planning a scope expansion or lease-time reduction, and 95%+ as urgent, since pools that run out of addresses cause new or renewing devices to fail to get network connectivity entirely.
How does DHCP lease time affect effective pool capacity?
Shorter lease times free up addresses from disconnected/idle devices faster, effectively increasing a pool's turnover rate and reducing the addresses tied up by stale leases — a busy pool nearing exhaustion can sometimes be relieved by shortening lease duration rather than expanding the range.
Can multiple DHCP scopes on different subnets share this calculation?
Each subnet's DHCP scope should be calculated independently using its own start/end range and reservation count — this calculator models a single scope; a multi-subnet environment needs one calculation per scope to get an accurate picture of overall capacity.