The link-local prefix (fe80::/10) is valid and unique only on a single link, comparable to IPv4’s auto-configuration addresses (169.254.0.0/16). Within this prefix, there’s a single subnet (fe80::/64). The last 64 bits are generated using stateless autoconfiguration. Every IPv6-enabled interface must have a link-local address, so applications can rely on its existence even without IPv6 routing.