Your Subnet Calculator Says /31 Has 0 Usable Hosts. RFC 3021 Says 2.

Mainstream subnet calculators ship four documented errors: /31 reports 0 usable hosts instead of 2 (RFC 3021), /32 reports 0 or -1 instead of 1, IPv6 panels render a fictional broadcast address field (RFC 4291 §2.1: IPv6 has no broadcast), and IPv6 point-to-point guidance still recommends /31 instead of /127 (RFC 6164). This one calculates by the RFCs, not by the 1995 textbook.