code: 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])
  }
docs: // parseIPv4Mask parses IPv4 netmask written in IP form (e.g. 255.255.255.0).
  // This function should really belong to the net package.