What does a CIDR prefix like /24 mean?

The prefix is how many bits from the left are the network portion. /24 means the first 24 bits are fixed (the network) and the remaining 8 bits address hosts, giving 256 addresses.

Why are usable hosts two fewer than total addresses?

In a normal subnet the first address is the network identifier and the last is the broadcast address, so they cannot be assigned to hosts. /31 and /32 are special cases used for point-to-point links and single hosts.

Is any data sent to a server?

No. All bitwise maths runs in your browser; nothing about the block is transmitted.

CIDR to IP Range

Expand a CIDR block to its network, broadcast and host range.

Turn a CIDR block into its full address range

Paste a block in CIDR notation (for example 10.0.0.0/22) and this tool computes the network address, broadcast address, the usable host range, the subnet mask, and the total and usable address counts.

All calculations are bitwise integer maths performed locally in your browser.