VLSM Allocation Validator
Check alignment, capacity, and overlaps.
VLSM Allocation Validator: 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 Planning, VLSM & CloudAll Tools
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Route Summary Calculator
Summarize networks and identify included gaps.
DHCP Scope Calculator
Plan allocation boundaries within a subnet.
DHCP Exclusion Calculator
Remove specified addresses and ranges.
How VLSM Allocation Validator Works?
The VLSM Allocation Validator works by running 3 checks on each planned subnet: boundary alignment, host capacity, and overlap with every other subnet.
Paste the plan
Enter each subnet with its CIDR and required host count.
Check alignment
Each network address must have every host bit set to 0.
Check capacity
Usable hosts of each prefix must meet the requirement.
Check overlaps
Each subnet is compared with every other subnet. The validator reports Pass or the failed check for every row.
VLSM Allocation Validator Formula
The VLSM Allocation Validator formula is Pass = Aligned AND (2^(32 − p) − 2 ≥ Hosts) AND No Overlap.
Alignment
(N AND Wildcard) = 0Network starts on its boundary.
Capacity
2^(32−p) − 2 ≥ HEnough usable hosts.
Overlap
S_i ≤ E_j AND S_j ≤ E_i → failChecked for each pair.
Worked example: 3 failing rows
Voice 192.168.20.100/26 → 100 mod 64 = 36 → not aligned Users 192.168.20.128/26 → 62 usable < 70 needed → too small Cameras 192.168.20.208/28 inside Printers 192.168.20.192/27 → overlap
VLSM Allocation Validator Conversion Table
The VLSM Allocation Validator conversion table validates a 6-row plan for 192.168.20.0/24 and shows the result of each check.
| Subnet | CIDR | Hosts Needed | Aligned | Capacity | Overlap | Result |
|---|---|---|---|---|---|---|
| Servers | 192.168.20.0/26 | 50 | Yes | OK (62) | No | Pass |
| Voice | 192.168.20.100/26 | 30 | No | OK (62) | No | Fail |
| Users | 192.168.20.128/26 | 70 | Yes | Short (62) | No | Fail |
| Printers | 192.168.20.192/27 | 20 | Yes | OK (30) | Yes | Fail |
| Cameras | 192.168.20.208/28 | 12 | Yes | OK (14) | Yes | Fail |
| WAN | 192.168.20.252/30 | 2 | Yes | OK (2) | No | Pass |
What is VLSM Allocation Validator?
The VLSM Allocation Validator is a tool that checks an existing VLSM plan for misaligned subnets, undersized subnets, and overlapping subnets.
Hand-built VLSM plans fail in 3 typical ways. A subnet starts on the wrong boundary, so the router reads a different network than the plan shows. A subnet is too small for its host count. 2 subnets claim the same addresses. The validator tests all 3 for every row.
The overlap check uses the network each device will actually compute. The Voice row at 192.168.20.100/26 resolves to 192.168.20.64/26, which collides with no other row, so the row fails on alignment alone.
What are the uses of VLSM Allocation Validator?
There are 4 main uses of the VLSM Allocation Validator, including design review, exam grading, and pre-deployment checks.
Reviewing network designs
Design reviews run the validator before approving an address plan.
Grading VLSM exercises
Instructors validate student plans in seconds.
Checking before deployment
Configuration templates are generated only from a passing plan.
Auditing inherited plans
Old spreadsheets are validated before reuse.