IPAddr#_ipv4_compat?

# File tmp/rubies/ruby-3.4.1/lib/ipaddr.rb, line 346
def _ipv4_compat?
  if !ipv6? || (@addr >> 32) != 0
    return false
  end
  a = (@addr & IN4MASK)
  return a != 0 && a != 1
end