Free Address Range Finder
Find gaps in entered allocation records.
Free Address Range Finder: 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.
Related IPv4 Subnetting & RangeAll Tools
IPv4 Range Calculator
Find address boundaries from a prefix.
IPv4 Subnet Calculator
Calculate subnet details from address and mask.
Network Address Calculator
Find the subnet’s network address.
IPv4 Broadcast Address Calculator
Find broadcast addresses where applicable.
First and Last Address Calculator
Identify a range’s endpoints.
Usable IPv4 Host Range Calculator
Apply the selected subnet-use rules.
How Free Address Range Finder Works?
The Free Address Range Finder works by sorting the entered allocation records, merging overlapping entries, and listing every stretch of the usable host range that no record covers.
Enter the subnet
Type the subnet to search, such as
192.168.30.0/24.Paste allocation records
Records can be single addresses (
192.168.30.150) or ranges (192.168.30.2-20).Merge and sort
The finder converts each record to an interval and merges overlapping or touching intervals.
List the free ranges
Each free range appears with its size, sorted from the lowest address.
Free Address Range Finder Formula
The Free Address Range Finder formula is Free = Usable Host Range − Merged Allocations.
Usable range
[Network + 1, Broadcast − 1]The search space for a standard subnet.
Merge rule
Start_(i+1) ≤ End_i + 1 → mergeJoins overlapping or adjacent records.
Free interval
[End_i + 1, Start_(i+1) − 1]Space between 2 merged records.
Free count
Σ (End − Start + 1)Total free addresses.
Worked example: 192.168.30.0/24
Records : .1, .2–.20, .50–.99, .150, .200–.254 Merged : .1–.20, .50–.99, .150, .200–.254 Free : .21–.49 (29), .100–.149 (50), .151–.199 (49) Total : 128 free addresses
Free Address Range Finder Conversion Table
The Free Address Range Finder conversion table lists the free ranges in 192.168.30.0/24 for the 5 allocation records in the worked example.
| Free Range | Addresses | As CIDR Blocks |
|---|---|---|
| 192.168.30.21 – 192.168.30.49 | 29 | 192.168.30.21/32, 192.168.30.22/31, 192.168.30.24/29, 192.168.30.32/28, 192.168.30.48/31 |
| 192.168.30.100 – 192.168.30.149 | 50 | 192.168.30.100/30, 192.168.30.104/29, 192.168.30.112/28, 192.168.30.128/28, 192.168.30.144/30, 192.168.30.148/31 |
| 192.168.30.151 – 192.168.30.199 | 49 | 192.168.30.151/32, 192.168.30.152/29, 192.168.30.160/27, 192.168.30.192/29 |
What is Free Address Range Finder?
The Free Address Range Finder is a tool that finds unused host addresses inside a subnet by comparing the subnet with a list of entered allocation records.
Allocation records come from DHCP leases, spreadsheets, and IPAM exports. The records mix single addresses and ranges, often with overlaps. The finder merges them first, so double-counted records never hide or invent free space.
The finder works at the address level inside 1 subnet. The CIDR Coverage Gap Calculator works at the block level inside a larger parent prefix.
What are the uses of Free Address Range Finder?
There are 4 main uses of the Free Address Range Finder, including static assignment, DHCP pool resizing, and cleanup.
Picking a free static address
The first free range gives an address that no record claims.
Resizing DHCP pools
Large free ranges show where a DHCP pool can grow.
Cleaning up spreadsheets
Overlapping records in the merge step reveal duplicate entries.
Preparing migrations
Free ranges in a target subnet show where migrated hosts can land.