The default value of ip6_v6only (sysctl net.inet6.ip6.v6only) has
been changed.  The new value brings us closer in line with current
RFC-defined behavior and practices.

Itojun still has significant concerns about the new default behavior.
His concerns have been well-documented in
ftp://ftp.itojun.org/pub/paper/draft-cmetz-v6ops-v4mapped-api-harmful-00.txt

Best Regards,
NetBSD OS PMC (core)