func parseIPv4Mask(s string) net.IPMask {
	mask := net.ParseIP(s)
	if mask == nil {
		return nil
	}
	return net.IPv4Mask(mask[12], mask[13], mask[14], mask[15])
}
func parseIPv4Mask(s string) net.IPMask {
	mask := net.ParseIP(s)
	if mask == nil {
		return nil
	}
	return net.IPv4Mask(mask[12], mask[13], mask[14], mask[15])
}
// parseIPv4Mask parses IPv4 netmask written in IP form (e.g. 255.255.255.0).
// This function should really belong to the net package.