CIDR Calculator

Convert an IPv4 start–end range into the minimal set of CIDR blocks (handy for firewall rules), or inspect an IPv6 prefix to see its first and last address and size.

Tool status: Fully functional Runs entirely on this website or in your browser.

IPv4 range → CIDR blocks

IPv6 prefix calculator

What this tool can and cannot do

Runs entirely in your browser.

Data sources: None – runs in your browser

Understanding the results

A range that does not align with a power of two needs several CIDR blocks to describe it exactly.

Troubleshooting

  • Make sure the start address is lower than the end address.

Frequently asked questions

How big is an IPv6 /64?

A /64 contains 2^64 ≈ 18.4 quintillion addresses – the standard size of a single IPv6 subnet.