IP Range to CIDR Converter

Network configuration often requires defining strict address blocks, yet specific requirements often come in the form of arbitrary ranges (e.g., 192.168.1.10 to 192.168.1.50). Firewalls, routers, and cloud security groups (like AWS or Azure) typically reject raw ranges, demanding standard CIDR (Classless Inter-Domain Routing) notation instead. Failing to convert these ranges correctly can leave gaps in security coverage or accidentally expose unintended addresses.

This tool eliminates that risk by mathematically decomposing any start-to-end IP range into the smallest possible set of valid CIDR blocks. It handles both IPv4 (32-bit) and IPv6 (128-bit) addresses, utilizing arbitrary-precision arithmetic to ensure bit-perfect accuracy. Whether you are migrating subnets or whitelisting specific IPs, this calculator ensures your network masks align perfectly with the binary boundaries of the Internet Protocol standards.

The core challenge is finding the largest possible power of two, 2^k, that satisfies two conditions: it must divide the current starting IP (binary alignment), and the resulting block must fit within the remaining range.

Once the optimal exponent k is found, the CIDR prefix length P is calculated based on the address bit-width (32 for IPv4, 128 for IPv6):

We then output the CIDR block IP_start/P and advance the start pointer: