Subnethero is a small C-based command-line utility that generates subnetting tables for flat networks, FLSM, and VLSM configurations. It doesn't try to be clever or pretty—just fast, predictable, and useful when you're staring down IP ranges and want something that doesn't get in your way.
If you're the kind of person who prefers raw output over bloated interfaces, and you think subnetting shouldn't require a spreadsheet or a web app, this might be worth a look.
The code is on GitHub. If it does what you need, consider starring the repo.
https://github.com/giovanni-iannaccone/subnethero