How I turn an IPv4 range into CIDR blocks

Many firewalls and cloud tools want CIDR blocks, but humans often receive IP ranges with a start and end address.

The IPv4 Range Expander tool helps me inspect the input and output without writing a one-off script first.

The simple idea

Many firewalls and cloud tools want CIDR blocks, but humans often receive IP ranges with a start and end address.

I use it to answer one focused question at a time, then I review the result before using it anywhere important.

Step 1: Enter the start IP address

Enter the start IP address.

Step 2: Enter the end IP address

Enter the end IP address.

Step 3: Generate the smallest covering CIDR blocks

Generate the smallest covering CIDR blocks.

Step 4: Review the number of blocks produced

Review the number of blocks produced.

Step 5: Check that the range does not include extra addresses you did not intend

Check that the range does not include extra addresses you did not intend.

Step 6: Use the CIDRs in firewall, routing, or allowlist configuration

Use the CIDRs in firewall, routing, or allowlist configuration.

Step 7: Document where the range came from

Document where the range came from.

My checklist

Before I trust the result, I check:

• Enter the start IP address.
• Enter the end IP address.
• Generate the smallest covering CIDR blocks.
• Review the number of blocks produced.
• Check that the range does not include extra addresses you did not intend.
• Use the CIDRs in firewall, routing, or allowlist configuration.
• Document where the range came from.

The tool makes the mechanical part faster. The decision still belongs to me: is this result correct, safe, and appropriate for the real task?