A group of researchers has conducted a rigorous comprehensive characterization of this DDoS attacks and of countermeasures to mitigate the associated risks. They gathered data from DDoS Protection Services (DPS), amplification honeypots, and a DNS measurement platform. One-third of all /24 networks recently estimated to be active on the Internet have suffered at least one DoS attack over the last two years. Google, GoDaddy, and Wix services host the most websites and also see the most attacks. The United States hosts around 25% of web addresses.”]
Source: http://securityaffairs.co/wordpress/65253/hacking/ddos-attack.html