Why does a range produce more than one CIDR block?

A single CIDR block only exists at addresses aligned to its own size — a /29 block, for example, can only start at a multiple of 8. If the start address of a range isn't aligned to a large enough power of two, or the range's length isn't itself a power of two, no single block covers it exactly, so the range is split into the fewest aligned blocks that together cover it precisely.

How is the minimal set of blocks computed?

Starting from the range's start address, the tool finds the largest CIDR block that can start there (based on how many trailing zero bits the address has in binary) without exceeding the remaining range, emits that block, advances past it, and repeats until the whole range is covered. This is the standard range-to-CIDR aggregation algorithm and always produces the fewest possible non-overlapping blocks.

Does this support IPv6 ranges?

No, this tool covers IPv4 ranges only. The underlying IP parsing helper this tool reuses doesn't yet expose a 128-bit integer form of IPv6 addresses, which the range-splitting arithmetic needs; entering an IPv6 address returns an explanatory message instead of a result.

What happens if the start and end IP are the same address?

A range of exactly one address is represented as that address with a /32 prefix, since /32 always covers exactly one host.

IP Range to CIDR Converter - Convert IP Range to CIDR Blocks

Overview

Introduction

Firewall rules, allow lists, and routing tables are usually expressed as CIDR blocks, but the range you actually need to cover — say, a block of addresses assigned by a hosting provider — often comes to you as a plain start and end IP instead.

This tool converts that start/end range into the smallest set of CIDR blocks that covers it exactly, entirely in your browser.

What Is IP Range to CIDR Converter?

A converter that takes an inclusive IPv4 address range (a start IP and an end IP) and returns the minimal list of non-overlapping CIDR blocks that together cover exactly that range — no more, no fewer addresses.

It's the reverse operation of this site's CIDR Subnet Calculator, which expands a single CIDR block into its host range; this tool goes the other way, aggregating a range back into CIDR notation.

How IP Range to CIDR Converter Works

Both addresses are converted to their 32-bit unsigned integer form. Starting from the range's start address, the tool repeatedly finds the largest power-of-two-sized block that can legally start at the current address (determined by counting trailing zero bits in its integer form) without extending past the end of the remaining range, emits that block as CIDR notation, and advances to the next unaddressed integer.

This greedy, alignment-driven approach is the standard algorithm for range-to-CIDR aggregation: at every step it picks the single largest valid block, which is proven to minimize the total number of blocks produced.

When To Use IP Range to CIDR Converter

Use it when a provider, spreadsheet, or ticket hands you an IP range as a start and end address and you need CIDR notation for a firewall rule, security group, or allow list.

It's also useful for double-checking that a hand-written set of CIDR blocks actually corresponds to the intended contiguous range, with nothing missing or double-covered.

Often used alongside CIDR Subnet Calculator and IP to Long Converter.

Features

Advantages

Always produces the minimal possible number of CIDR blocks for a given range, not just any valid covering set.
Reuses this site's shared IPv4 parsing and integer-conversion logic, so validation behavior matches the other IP converters here.
Reports the total address count alongside the block list, useful for a quick sanity check against the expected range size.

Limitations

IPv4 only; IPv6 ranges aren't supported since the underlying integer math would need a 128-bit (BigInt) representation not currently wired up for IPv6 in this tool's shared helpers.
Caps output at 512 CIDR blocks to keep results readable; a pathologically unaligned, very large range can need more blocks than that and should be split into smaller sub-ranges first.

Examples

A range that's already block-aligned

Input

10.0.0.0 to 10.0.0.7

Output

10.0.0.0/29

8 addresses starting at an address aligned to a multiple of 8 fit exactly into a single /29 block.

A range that isn't aligned to any single block

Input

10.0.0.1 to 10.0.0.4

Output

10.0.0.1/32
10.0.0.2/31
10.0.0.4/32

10.0.0.1 can only start a /32 (it's an odd address), 10.0.0.2 can start a /31 covering .2-.3, and the remaining single address .4 needs its own /32 — three blocks in total, the minimum possible for this exact range.

Best Practices & Notes

Best Practices

Check the reported total address count against what you expected before using the blocks in a firewall rule — a mismatch usually means the start or end IP was mistyped.
When a range produces many small blocks, consider whether widening the range slightly (if the extra addresses are harmless to include) would let it collapse into fewer, larger blocks.
Feed the resulting blocks into a tool like this site's Firewall Rules Generator or IP Allow List Generator rather than retyping them by hand.

Developer Notes

IPv4-to-integer and integer-to-IPv4 conversions are reused directly from ip-core.ts's ipToLong/longToIp rather than reimplemented, so validation (octet range, leading zeros) stays consistent with the other converters in this category. The block-splitting loop finds each block's maximum size via trailing-zero-bit counting on the current address (how large a power-of-two block can start there) capped by an integer loop against the remaining range length rather than Math.log2, avoiding floating-point rounding at large powers of two.

IP Range to CIDR Converter Use Cases

Converting a hosting provider's assigned IP range (given as start/end addresses) into CIDR blocks for a security group or firewall rule
Turning a spreadsheet of IP ranges into CIDR notation for a routing table or allow list
Verifying that a manually assembled set of CIDR blocks exactly covers an intended contiguous range

Common Mistakes

Entering the end IP before the start IP, which this tool rejects since the range would be empty or reversed.
Expecting a single CIDR block for every range; only ranges whose start address and length are both power-of-two aligned collapse into one block.

Tips

If you only need to double-check whether a specific address falls inside the range, it's simpler to compare it directly against the start and end IPs than to check it against every output block.

IP Range to CIDR Converter