IPv4 Range to CIDR Converter

This tool converts a start IPv4 address and an end IPv4 address into the smallest set of CIDR blocks that exactly covers the requested range. The form and output stay exactly where they were, but the new content explains why an address span sometimes maps to one prefix and sometimes needs several smaller prefixes.

That distinction matters in firewall policies, routing summaries, cloud object groups, and migration planning. If a range is already aligned to a valid network boundary, the result may be a single CIDR. If the boundaries are uneven, the converter builds a minimal list of prefixes so you do not over-include addresses outside the requested range.

Inputs explained

The start IP is the first IPv4 address that must be included. The end IP is the last IPv4 address that must be included. They can be any valid IPv4 addresses as long as the end is not smaller than the start. The calculator then walks the span from left to right, choosing the largest aligned CIDR block that fits at each step.

How it works

The method is a standard minimal-cover algorithm. Starting from the first address, the tool checks alignment and remaining range size, picks the largest prefix that does not overshoot the end, outputs that block, then repeats until every address in the requested span is covered.

IP Range → CIDR Converter

Start IP

End IP

CIDR Blocks

Result

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Step-by-Step Example

Enter 192.168.1.10 as the start and 192.168.1.30 as the end. That span is not aligned to a single power-of-two boundary, so one CIDR block cannot represent it exactly. The tool therefore breaks the range into several smaller prefixes that fit tightly around the requested addresses.

For a clean aligned range such as 192.168.1.0 through 192.168.1.255, the result is simply 192.168.1.0/24. The difference between those two examples shows why range conversion is not just notation swapping. It is a boundary alignment problem.

Use Cases

Use this page when you receive raw start and end IPs in a ticket, spreadsheet, or abuse report and need a concise prefix list for routers, firewalls, or documentation. It is also useful when you want to compress large allowlists into fewer entries while preserving an exact match to the requested address span.

Assumptions and limitations

The output is an informational exact cover for the range entered. It does not check ownership, address reputation, or whether a broader summary would be acceptable for your policy. Review special-purpose space, private space, and operational scope before applying the result in production.

Frequently Asked Questions

What does range to CIDR conversion do?

It turns a start IPv4 address and an end IPv4 address into the smallest practical list of CIDR blocks that covers the range without leaving gaps.

Why can one IP range produce multiple CIDR blocks?

A single CIDR works only when the range aligns to a power of two boundary. Unaligned ranges need multiple prefixes to cover the exact start and end points.

Does the tool require network aligned inputs?

No. You can enter any valid start and end IPv4 addresses as long as the end address is not lower than the start address.

When is minimal CIDR output useful?

Minimal CIDR sets are useful for route aggregation, ACL object groups, IP allowlists, documentation, and any workflow where a shorter prefix list is easier to manage.

Can the result be used directly in firewall rules?

Usually yes, but you should still review vendor syntax, object limits, and policy intent before applying the generated prefixes in production.

What limitations should I keep in mind?

The output is an exact mathematical cover for the range entered. It does not check business ownership, special-purpose space, or whether a broader prefix would create unintended matches.