Next Available Subnet Finder
Find aligned space within a parent.
Next Available Subnet 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
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IPv4 Subnet Calculator
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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 Next Available Subnet Finder Works?
The Next Available Subnet Finder works by stepping through a parent block at the requested prefix size and returning the first aligned block that overlaps no existing allocation.
Enter the parent block
Type the block to search, such as
10.60.0.0/20.Enter the existing allocations
Paste the allocated subnets, one per line.
Enter the requested size
Pick a prefix, such as
/24.Scan aligned candidates
The finder tests
10.60.0.0/24,10.60.1.0/24, and onward, and returns the first block with no overlap.
Next Available Subnet Finder Formula
The Next Available Subnet Finder formula is Result = the first k where Candidate_k = Parent + k × 2^(32 − r) overlaps no allocation, with r the requested prefix.
Candidate block
C_k = N + k × 2^(32−r)Always aligned to /r.
Overlap test
Start_C ≤ End_A AND End_C ≥ Start_ARuns against every allocation A.
Candidates scanned
k = 0 … 2^(r − p) − 1p is the parent prefix.
Worked example: /24 from 10.60.0.0/20
Used: 10.60.0.0/24, 10.60.1.0/25, 10.60.2.0/23, 10.60.4.0/22 10.60.0.0/24 → used 10.60.1.0/24 → overlaps 10.60.1.0/25 10.60.2.0 – 10.60.7.0 /24 → used Result → 10.60.8.0/24
Next Available Subnet Finder Conversion Table
The Next Available Subnet Finder conversion table shows the first free aligned block in 10.60.0.0/20 for 7 requested sizes, using the 4 allocations from the worked example.
| Requested Prefix | Block Size | Next Available Subnet |
|---|---|---|
| /21 | 2,048 | 10.60.8.0/21 |
| /22 | 1,024 | 10.60.8.0/22 |
| /23 | 512 | 10.60.8.0/23 |
| /24 | 256 | 10.60.8.0/24 |
| /25 | 128 | 10.60.1.128/25 |
| /26 | 64 | 10.60.1.128/26 |
| /27 | 32 | 10.60.1.128/27 |
What is Next Available Subnet Finder?
The Next Available Subnet Finder is a tool that returns the lowest free subnet of a requested size inside a parent block, aligned to its own prefix boundary.
Free space alone is not enough. A /24 must start on a /24 boundary, so a free half of 10.60.1.0/24 cannot host a /24. The finder checks alignment and overlap together and skips any candidate that fails either check.
Smaller requests reuse leftover fragments. In the example, a /25 lands at 10.60.1.128/25, which fills the space next to an existing /25 instead of opening new space at 10.60.8.0.
What are the uses of Next Available Subnet Finder?
There are 4 main uses of the Next Available Subnet Finder, including IPAM allocation, cloud provisioning, and fragment reuse.
Allocating subnets in IPAM
Requests for "the next /26 in the campus block" get an exact answer.
Provisioning cloud subnets
Automation finds a free CIDR inside a VPC before creating a new subnet.
Reusing small fragments
Small requests fill leftover space first, which keeps large blocks intact.
Checking parent capacity
A "None free" result shows that the parent block is full at that size.