Web site host and domain name registrar Register.com has been the target of a sustained attack this week, disrupting service for thousands of customers. The attacks began on Wednesday, causing a three-hour outage for many Web sites that rely on the company for hosting and/or use the company’s domain name system (DNS) servers, said Roni Jacobson, executive vice president at Register.com. The outage was the result of what’s known as a distributed denial of service (DDoS) attack, in which attackers cause hundreds or thousands of compromised PCs to flood a target with so much junk traffic that the Web site can no longer accommodate legitimate visitors. Typically, DDoS attacks are waged as a way for criminals to extort money from the targets, who are told the attack will cease when a ransom demand is paid. Jacobson declined to say whether Register.com had received any extortion demands. “We did have a