You're confusing ULAs (Unique Local Addresses) with LLAs (Link-Local Addresses). (ULAs don't need the interface specified.) ULA: fc..:… and fd..:… LLA: fe80:

You're confusing ULAs (Unique Local Addresses) with LLAs (Link-Local Addresses). (ULAs don't need the interface specified.) ULA: fc..:… and fd..:… LLA: fe80:… [ed.: By the way, sin6_scope_id is where the interface identifier is stored in struct sockaddr_in6. So, basically every single IPv6 address object you're handling has the field for it.]