Enter an IP address with its CIDR prefix into a browser-based subnet calculator and it returns the network address, the broadcast address and the usable host range. It runs entirely on your device, so nothing about your network is sent anywhere.
Inside any subnet, the very first address is the network address that identifies the block, and the very last address is the broadcast address used to reach every host at once. Neither one can be assigned to a device, so knowing them tells you exactly which addresses are actually usable. A subnet calculator derives both from the IP and the CIDR prefix using bitwise math, which is easy to get wrong by hand.
A client-side subnet calculator does the arithmetic in your browser, so your internal addressing plan is never uploaded to a server. That matters when the network you are documenting is private infrastructure. It is free, needs no account, and gives you the network address, broadcast address, mask and host count in one step.
No. The calculation runs entirely in your browser, so nothing is sent to a server.
No. Those two addresses are reserved, so the usable host range sits between them.
Yes. You can enter any valid prefix and it returns the matching network and broadcast addresses.