DHCP Scope Calculator
Plan allocation boundaries within a subnet.
DHCP Scope 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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DHCP Exclusion Calculator
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How DHCP Scope Calculator Works?
This tool works by taking the usable host range of a subnet, removing a block for the gateway and static devices at the start, removing a buffer at the end, and returning the remaining range as the DHCP scope.
Enter the subnet
Type the subnet, such as
10.30.40.0/23.Set the static reserve
Choose how many addresses to keep at the start for the gateway and static devices, such as 20.
Set the end buffer
Choose how many addresses to keep at the end, such as 10.
Return the scope
The calculator returns the start, end, size, and the matching DHCP options: router, subnet mask, and broadcast.
DHCP Scope Calculator Formula
The formula is Scope Start = Network + 1 + Static Reserve, Scope End = Broadcast − 1 − End Buffer.
Scope start
Network + 1 + RR addresses reserved after the gateway position.
Scope end
Broadcast − 1 − BB addresses reserved at the top.
Scope size
(2^(32−p) − 2) − R − BLeasable addresses.
Option 3 (router)
Network + 1Common gateway convention.
Worked example: 10.30.40.0/23, reserve 20, buffer 10
Usable = 10.30.40.1 – 10.30.41.254 (510) Start = 10.30.40.0 + 1 + 20 = 10.30.40.21 End = 10.30.41.255 − 1 − 10 = 10.30.41.244 Size = 510 − 20 − 10 = 480 leases
DHCP Scope Calculator Conversion Table
The table builds a scope for 7 subnet sizes, using a static reserve of 20 addresses and an end buffer of 10 addresses.
| Subnet | Gateway | Scope Start | Scope End | Leases | Mask (Option 1) |
|---|---|---|---|---|---|
| 10.30.40.0/23 | 10.30.40.1 | 10.30.40.21 | 10.30.41.244 | 480 | 255.255.254.0 |
| 192.168.1.0/24 | 192.168.1.1 | 192.168.1.21 | 192.168.1.244 | 224 | 255.255.255.0 |
| 192.168.2.0/25 | 192.168.2.1 | 192.168.2.21 | 192.168.2.116 | 96 | 255.255.255.128 |
| 172.16.8.0/22 | 172.16.8.1 | 172.16.8.21 | 172.16.11.244 | 992 | 255.255.252.0 |
| 10.0.0.0/21 | 10.0.0.1 | 10.0.0.21 | 10.0.7.244 | 2,016 | 255.255.248.0 |
| 10.200.0.0/20 | 10.200.0.1 | 10.200.0.21 | 10.200.15.244 | 4,064 | 255.255.240.0 |
| 192.168.50.0/26 | 192.168.50.1 | 192.168.50.21 | 192.168.50.52 | 32 | 255.255.255.192 |
What is DHCP Scope Calculator?
The DHCP Scope Calculator is a tool that plans the start, end, and size of a Dynamic Host Configuration Protocol (DHCP) scope inside a subnet.
A DHCP scope must stay inside the usable host range and away from statically assigned devices. The most common layout keeps the gateway and infrastructure at the low end, leaves a small buffer at the high end, and gives the middle to DHCP.
The calculator outputs the values that Windows DHCP Server, ISC Kea, dnsmasq, and router DHCP services ask for: scope range, subnet mask (option 1), router (option 3), and broadcast address (option 28).
What are the uses of DHCP Scope Calculator?
There are 4 main uses of the DHCP Scope Calculator, including scope creation, site templates, and Wi-Fi planning.
Creating DHCP scopes
Administrators copy the start, end, and options into the DHCP server.
Standardizing site templates
Every site uses the same reserve and buffer values.
Planning Wi-Fi pools
Guest Wi-Fi scopes on /22 or /21 subnets need exact lease counts.
Avoiding static conflicts
The reserved block keeps DHCP away from printers and servers.