From KingFisher Labs
IP Calc
Every number a subnet holds: networks, netmasks, hosts, and the addresses AWS, Azure, and Google Cloud reserve. All of it offline, with no account.
How it works
Drag the prefix; the numbers follow.
Type an address and drag the prefix: network, broadcast, netmask, usable range, and host count update as you move. The address space itself is the instrument. Drag it to resize a block, and step to the blocks on either side from keys that name where they land.
For cloud work, switch profiles to apply the IPv4 reservation rules for AWS, Azure, or Google Cloud and read the usable count those providers actually leave you. Each reserved address is named rather than just subtracted.
Nine tools round out the app: VLSM, subnet split, compare, supernet, IPv6, bandwidth, converter, wildcard/ACL, and validator. Bookmark the blocks you care about, and the Saved screen checks them against each other for overlap. Everything runs on the phone: no account and no internet connection needed.