No broadcast address, and why /64 is the usual floor

IPv4 subnetting habits don't transfer directly to IPv6 in a couple of important ways. There's no broadcast address in IPv6 at all — multicast replaces that role entirely — so every address other than the network (all-zeros) address in a block is a plain usable address, not "usable minus two" the way an IPv4 /24 works out. The other habit that doesn't transfer is subnetting down tightly: /64 is the conventional minimum for any subnet that needs to host actual devices, because SLAAC (stateless address autoconfiguration) and several other IPv6 mechanisms are specified to work with exactly 64 bits of host address space — carving a /64 block into smaller pieces breaks those mechanisms for anything inside it, even though the address math to do so is perfectly valid.

Why this needs BigInt

A 128-bit address is far larger than JavaScript's native Number type can represent exactly (doubles lose integer precision past 2^53), so any IPv6 tool that does real address-range math — finding a network address, computing the last address in a block, counting total addresses — has to use BigInt arithmetic throughout rather than the 32-bit bitwise tricks that work fine for IPv4. That's the entire reason this is a separate tool from the IPv4 subnet calculator rather than one tool that handles both: the underlying math is genuinely different, not just a bigger version of the same thing.