CIDR Gap Finder

The CIDR Gap Finder identifies unallocated address space within a parent CIDR block by comparing it against a list of existing subnet allocations. Enter a parent range like 10.0.0.0/16 and your current allocations to instantly discover every free gap available for new deployments, showing each gap as a CIDR block with its size and position.

Understanding IP Address Space Gaps

In any managed network, address space gets consumed incrementally. You allocate a /24 for the server VLAN, another for desktops, a /27 for the DMZ, a few /28 blocks for management interfaces, and before long your parent block looks like Swiss cheese. The problem is not that you are running out of space -- most organizations use a fraction of their allocated range -- but that you have lost track of where the free space actually is. Finding a contiguous block large enough for your next deployment means mentally walking through the allocation map and doing arithmetic on subnet boundaries, which is exactly the kind of work that leads to mistakes.

This is where gap analysis becomes essential. Instead of eyeballing a spreadsheet or scrolling through an IPAM tool trying to spot unused ranges, you feed the gap finder your parent CIDR and everything currently allocated within it. The tool then reports every gap: its position, its size in CIDR notation, and how many addresses it contains. You get a complete picture of your free space in seconds, not hours.

Gap analysis is also the first step in addressing fragmentation. If your free space is scattered across dozens of small gaps rather than concentrated in a few large blocks, you may need to renumber some subnets to consolidate free space before you can fit a large new allocation. The gap finder makes fragmentation visible at a glance by showing you the size distribution of your gaps, the largest contiguous block available, and the overall utilization percentage of your parent range.

How to Find Unallocated IP Space in a CIDR Block

1. Enter the parent CIDR block that defines your total address space (e.g., 10.0.0.0/16)
2. Enter all existing allocations within that parent, one per line (e.g., 10.0.0.0/24, 10.0.1.0/24, 10.0.10.0/24)
3. Click analyze to scan for gaps
4. Review each unallocated range with its CIDR notation, address count, and position within the parent
5. Use the results to plan your next subnet allocation or assess fragmentation

Gap Finder Examples

Finding gaps in a /16 with scattered /24 allocations

Parent: 10.0.0.0/16

Existing allocations:

10.0.0.0/24
10.0.1.0/24
10.0.2.0/24
10.0.10.0/24
10.0.20.0/24

Gaps found:

Summary: 63,256 of 65,536 addresses free (96.5% available)

Finding gaps in a small /24

Parent: 192.168.1.0/24 Allocations: 192.168.1.0/26, 192.168.1.128/26

When to Use the CIDR Gap Finder

• IPAM capacity planning -- determine how much free address space remains before requesting a new allocation from your RIR or provider
• Subnet provisioning -- find where a new /24 or /27 can fit within your existing address plan
• Fragmentation analysis -- identify whether free space is concentrated in large contiguous blocks or scattered in unusable fragments
• Compliance audits -- verify that all address space within an allocated block is accounted for and documented
• Data center migrations -- map available address space in the target environment before moving workloads
• Mergers and acquisitions -- assess the address space inventory of a newly acquired network before integration planning

How IP Address Space Gap Detection Works

The gap finder operates through a straightforward algorithm:

Sorting and Normalization

All input allocations are first normalized to canonical CIDR form and sorted by their starting IP address numerically. Overlapping or duplicate allocations are merged so they don't produce false gaps.

Linear Scan

Starting from the first address of the parent CIDR, the algorithm walks forward through each allocation. Any address range between the end of one allocation and the start of the next is a gap. The final gap (if any) extends from the last allocation to the end of the parent range.

Parent:      |================================|
Allocations: |AAAA|    |BBB|     |CC|         |
Gaps:        |    |XXXX|   |XXXXX|  |XXXXXXXXX|

CIDR Decomposition

Each gap is expressed as the minimal set of CIDR blocks that cover it exactly. A gap from 10.0.3.0 to 10.0.7.255 becomes 10.0.3.0/24, 10.0.4.0/22 -- the fewest CIDR blocks that tile the space without overlap.

Aggregate Statistics

The tool calculates total free space, number of distinct gaps, largest and smallest gaps, and overall utilization percentage. These metrics help you quickly assess whether your address plan has room to grow or needs restructuring.

Practical Advice for Address Space Management

When reviewing gap analysis results, pay attention to the largest contiguous gap, not just the total free space. Having 10,000 free addresses spread across 50 small gaps is very different from having them in two or three large blocks. If your largest gap is a /27 (32 addresses) but you need a /24 (256 addresses), you cannot deploy without renumbering, even though the total free space might seem ample.

A good rule of thumb for address planning is to allocate subnets from the beginning of your parent block and leave free space at the end. This "sequential packing" strategy keeps your gaps contiguous and makes future allocations predictable. When you inherit a fragmented address plan, use the gap finder to identify the largest free blocks and prioritize allocating from those rather than filling every tiny gap. Over time, as subnets are decommissioned, you can reclaim and consolidate space.

Run gap analysis regularly, not just when you need new space. Quarterly reviews catch problems early: subnets that were allocated for temporary projects and never reclaimed, test environments that have grown beyond their original allocation, or address ranges that have been informally reserved but never documented. The gap finder turns these hidden issues into visible data that you can act on during maintenance windows rather than in the middle of an urgent deployment.

Frequently Asked Questions

What if allocations overlap each other?

Overlapping allocations are merged automatically before gap analysis. If 10.0.0.0/23 and 10.0.1.0/24 both appear in your list, the tool recognizes the /24 is already covered by the /23 and treats them as a single allocation spanning 10.0.0.0/23.

Can I filter gaps by minimum size?

Yes. Set a minimum prefix length or minimum address count to only display gaps large enough for your planned allocation. This is useful when you need a /24 (256 addresses) and want to ignore smaller fragments that cannot fit your requirement.

Does this tool handle IPv6 gap analysis?

Yes. The gap finder works with both IPv4 and IPv6 CIDR blocks. For IPv6, even a /64 parent contains 2^64 addresses, so the tool reports gap sizes using prefix notation rather than raw address counts for readability.

What is the difference between a gap and unallocated space?

In this context, they mean the same thing. A gap is any address range within the parent CIDR that is not covered by any of the listed allocations. If an allocation exists but you did not include it in the input, it will appear as part of a gap. Make sure your allocation list is complete for accurate results.

Can I use this tool to find gaps across multiple parent blocks?

The tool analyzes one parent CIDR at a time. If you manage multiple non-contiguous address blocks, run the gap finder separately for each parent. For a unified view of all your address space, consider using an IPAM system or running gap analysis on a supernet that covers all your blocks.

Related Tools

• Next Available Subnet Finder - Find the next available subnet within a parent block
• Subnet Allocation Planner - Plan subnet allocations to fit requested blocks
• CIDR Exclusion Tool - Subtract exception CIDRs from a parent block
• Wasted Space Estimator - Calculate wasted address space in allocations
• CIDR Aggregator - Aggregate multiple CIDR blocks into summarized routes