Subnet takes an IPv4 address and a CIDR prefix and works out everything about the network — the network and broadcast addresses, the usable host range, the subnet mask and wildcard, and how many hosts fit. Type an address like 192.168.1.10 and a prefix like 24, or paste 192.168.1.10/24 straight in. Nothing is uploaded.
Calculations follow standard IPv4 rules. A /31 is treated as a point-to-point link (2 usable addresses, per RFC 3021) and a /32 as a single host; every other block reserves the network and broadcast addresses, so usable hosts are two fewer than the total. Everything is computed in your browser — nothing is uploaded.