Is this subnet calculator really free?

Yes. Every tool runs entirely in your browser, there's no signup, and your inputs never leave your device.

What's the difference between a subnet mask and a wildcard mask?

A subnet mask marks network bits with 1s and host bits with 0s (e.g. 255.255.255.0). A Cisco ACL wildcard mask is its inverse (0.0.0.255) — 0 bits must match, 1 bits are ignored. The Wildcard / ACL tab converts between them automatically.

What is VLSM and why does the boundary matter?

Variable Length Subnet Masking lets you carve subnets of different sizes from one address block instead of wasting addresses on equal-sized subnets. Getting a boundary wrong causes two subnets to overlap, which the VLSM Planner and Compare tabs both catch.