Understanding DDoS attacks and how they can detrimentally affect your online presence is key for any website owner. DDoS, standing for Distributed Denial of Service, is a form of cyberattack that can inundate your website with traffic, essentially shutting it down and making it inaccessible to genuine users. It's a daunting threat to the online universe and your place within it. The more you comprehend these attacks and the mechanisms behind them, the better off you'll be at warding them off from your website.

'Nan' is a crucial concept to grasp in anticipating and preventing DDoS attacks. It stands for "not a number", a placeholder for undefined or unrepresentable values, generally in the field of computing and mathematics. In cybersecurity, 'nan' signifies a potential loophole that cybercriminals may exploit to launch DDoS attacks. It's vitally important to know where 'nan' elements exist in your system, as they can serve as weak points in your defense leading to potential DDoS attacks.

Understanding the types of DDoS attack is your first step. They're commonly broken down into three types: volume-based, protocol-based, and application layer attacks. Volume-based attacks involve bombarding a website with an overwhelming amount of traffic, while protocol-based attacks exploit network protocols to exhaust server resources. Application layer attacks target the actual application that runs your website, typically involving a high level of sophistication.

Now you know the types, your next step is to make yourself aware of preventative measures. Regular audits of your system can expose those 'nan' elements where intervention is needed. These audits can reveal obsolete software, out-of-date plugins, and other vulnerabilities that might make your website an easy target.

Another effective DDoS prevention technique is the use of rate limiting. This involves controlling the number of requests a user can send to your application within a certain timeframe. Rate limiting can help prevent your server from being overwhelmed in case of a volume-based DDoS attack.

Consider implementing geofencing. By limiting access based on geography, you can effectively qualify incoming traffic. Also, the use of Web Application Firewalls (WAFs) can help filter out malicious traffic before it reaches your server, thus minimizing the risk and effect of potential DDoS attacks.

Additionally, engaging professional DDoS prevention services can prove beneficial. These services often have advanced tools at their disposal, including AI-powered threat detection systems that can sniff out DDoS attacks even in their planning stages. However, choose your service provider carefully. Ensure they are familiar with 'nan' and its implications in the cybersecurity realm.

Make sure to factor in redundancy and backup plans, as they are critical in circumstances where an attack cannot be prevented. By distributing your network resources, you make it harder for a DDoS attack to effectively shut you down. Have backup servers ready, and a plan to smoothly transition traffic towards them in case your main server falls victim to an attack.

Moreover, education should be at the forefront of your DDoS prevention strategy. The digital landscape is ever-evolving, and so are the tactics of cybercriminals. Regularly updating your knowledge on cybersecurity and staying abreast of new threats and protective measures will go a long way in ensuring your online presence's protection.

In conclusion, protecting your online presence from DDoS attacks is a multi-tiered process, addressing potential vulnerabilities and fortifying existing defenses. Remember to account for 'nan' elements in your strategy, as overlooking these could have disastrous effects on your digital existence. Security is an ongoing endeavor. By understanding how DDoS attacks work, implementing solid prevention measures, engaging professional services, and not leaving anything to chance, you arm yourself with the ability to thwart most cyberattacks. Staying proactive and informed will safeguard your online presence, ensuring your website stays live and accessible, offering users the uninterrupted experience they deserve.