public function hasNext() {
        if (!isset($this->current)) {
            throw new BadMethodCallException('You need to call method "load()" first.');
        }

        for ($ipLong = $this->current; $ipLong <= $this->last; $ipLong++) {
            if ($this->isUsed($ipLong) === false) {
                return true;
            }
        }

        return false;
    }