What counts as an overlap?

Any two entries that share at least one address. For aligned CIDR blocks that means one contains the other or they are identical; bare IPs overlap a block when they fall inside it. The report names the exact relationship for each conflicting pair.

Why does this matter for firewall and NSG rules?

Overlapping CIDRs in an allow/deny list create shadowed or ambiguous rules — a broader rule can mask a narrower one, or two rules can contradict. Surfacing the overlaps lets you spot duplicates and ordering problems before they cause an outage or a security gap.

Can I label my entries?

Yes. Put a label after the CIDR on the same line (e.g. 10.0.0.0/24 prod-web) and it is carried into the overlap report so you can tell which named rules collide.

CIDR Overlap Checker

About CIDR Overlap Checker

Checks a list of IPv4 CIDR blocks and addresses for overlaps — the conflicts that cause shadowed firewall rules, ambiguous routes, and duplicate security-group entries. Every pair of blocks that shares any address is reported, with the exact relationship (identical/duplicate, one fully contains the other, or partial overlap) and your own labels carried through so you can see which rules collide. Blocks that do not share any address are correctly reported as disjoint.

Compares every pair of entries by address range, not just by text
Classifies each overlap: identical/duplicate, A-contains-B, B-contains-A, or partial
Accepts bare IPs (treated as /32) and CIDR blocks /0–/32, with optional per-line labels
Labels are preserved in the report so colliding rules are easy to identify
Invalid lines are skipped and listed rather than failing the whole batch
Runs entirely in the browser — no addresses leave your machine

Frequently Asked Questions

What counts as an overlap?: Any two entries that share at least one address. For aligned CIDR blocks that means one contains the other or they are identical; bare IPs overlap a block when they fall inside it. The report names the exact relationship for each conflicting pair.
Why does this matter for firewall and NSG rules?: Overlapping CIDRs in an allow/deny list create shadowed or ambiguous rules — a broader rule can mask a narrower one, or two rules can contradict. Surfacing the overlaps lets you spot duplicates and ordering problems before they cause an outage or a security gap.
Can I label my entries?: Yes. Put a label after the CIDR on the same line (e.g. 10.0.0.0/24 prod-web) and it is carried into the overlap report so you can tell which named rules collide.

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Body Surface Area Calculator — Calculate body surface area from height and weight.
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Electrical Power Calculator — Calculate watts, volts, amps, and electricity costs.
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Fabric Calculator — Calculate how much fabric you need for a project.
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Fuel Economy Calculator — Calculate MPG or L/100km from distance and fuel used.
Graphing Calculator — Plot mathematical functions and equations on a graph.
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Ohms Law Calculator — Calculate voltage, current, resistance, or power using Ohm's law.
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Paint Calculator — Calculate how much paint you need for walls or rooms.
Percentage Calculator — Find percentages, percent change, and what percent X is of Y.
Pregnancy Calculator — Calculate due date and pregnancy milestones.
Recipe Nutrition Calculator — Calculate calories and macros for a recipe.
Resistor Color Code Calculator — Decode resistor values from color bands.
Solar System Sizing Calculator — Size a solar panel system for your energy needs.
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Surround Sound Setup Calculator — Calculate speaker placement for surround sound.
Time Zone Calculator — Convert times between different time zones.
Unit Price Calculator — Compare prices per unit to find the best deal.

Converters

Binary Text Converter — Convert text to binary and binary to text.
CIDR to IP Range Converter — Convert CIDR notation to IP address ranges.
Color Code Converter — Convert colors between hex, RGB, HSL, and CMYK.
CSV to JSON Converter — Convert CSV data to JSON format.
Data Storage Converter — Convert between bytes, KB, MB, GB, TB, and more.
JSON to CSV Converter — Convert a JSON array of objects to CSV — nested keys flattened, columns merged, values RFC-4180 escaped.
JSON to YAML Converter — Convert between JSON and YAML in both directions, with auto-detection — for configs, pipelines, and manifests.
Markdown to HTML Converter — Convert Markdown to HTML.
Number Base Converter — Convert numbers between binary, octal, decimal, hexadecimal, and any custom base 2–36.
Roman Numeral Converter — Convert between Roman numerals and regular numbers.
Time Converter — Convert between hours, minutes, seconds, and more.
Unit Converter — Convert length, weight, temperature, and other units.
URL Encoder Decoder — Encode and decode URL strings.

Date & Time

Countdown Timer — Count down to any date or event.
Stopwatch — Stopwatch with lap times.
World Clock — See current time in cities around the world.

Development

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JSON Formatter — Prettify, minify, and validate JSON.
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Regex Pattern Explainer — Explain what a regex pattern does in plain English.
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XML Formatter & Validator — Format, minify, and validate XML with error highlighting.
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Educational

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Typing Speed Test — Test your typing speed and accuracy.

Finance

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Depreciation Calculator — Calculate asset depreciation with yearly schedules.
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Gaming

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Gardening

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Gardening Planner — Plan garden layouts and planting schedules by zone.

Generators

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Character Name Generator — Generate random character names by genre and origin.
Citation Generator — Generate citations in APA, MLA, Chicago, and other formats.
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