Function is_ipv6_transition_mechanism 

pub fn is_ipv6_transition_mechanism(ip: Ipv6Addr) -> bool

Returns true for IPv6 transition-mechanism prefixes that embed an IPv4 address reachable through the mechanism — NAT64 (64:ff9b::/96), 6to4 (2002::/16), and Teredo (2001::/32). These are more credible SSRF bypasses than IPv6 documentation ranges and are blocked wholesale (not selectively unwrapped) since this is a conservative preflight for an OAuth Authorization Server fetching an attacker-supplied URL, not a general-purpose network client that needs transition-mechanism support.