Subnets · free
IP range to CIDR
Turn any start–end range into the exact list of CIDR blocks.
447 addresses in 10 blocks:
Firewalls, cloud security groups and allow-lists usually want CIDR blocks, while registries and documentation often give a start and end address. A range that does not line up with powers of two needs several blocks: the converter finds the smallest set that covers exactly the range, nothing more.
The regional registries publish their allocations as a start address and a count. The country IP range lists on this site are converted to CIDR exactly this way.
Why does one range turn into many blocks?
A CIDR block always holds a power of two addresses and must start at a multiple of its size. 10.0.0.1 to 10.0.0.10, for example, needs 10.0.0.1/32, 10.0.0.2/31, 10.0.0.4/30, 10.0.0.8/31 and 10.0.0.10/32.