public String toString() {
    	if(bits == null)
    		throw new IllegalStateException();
    	String mask = new String();
    	String space = new String();
    	for(int i = bits.length - 1; i >= 0; i--) {
    		space = "";
    		if((i + 1) % 8 == 0 && i < bits.length - 1)
    			space = " ";
    		mask += space + bits[i];
    	}
    	return mask;
    }