public function getDelta(): IP
    {
        if ($this->delta === null) {
            if ($this->prefix == 0) {
                $this->delta = new static::$ip_class(static::$ip_class::MAX_INT);
            } else {
                $this->delta = new static::$ip_class(gmp_sub(gmp_shiftl(1, static::$ip_class::NB_BITS - $this->prefix), 1));
            }
        }

        return $this->delta;
    }