According to Netcraft Ltd., a Bath, England-based Internet performance monitoring company, more than 550,000 domains are registered with Joker. 'Any of those domains that use Joker's DNS servers are likely to be affected,' Netcraft said in a note posted on its Web site.

A DNS server is used to look up and translate domain names such as Computerworld.com from plain text to numeric Internet Protocol addresses.

Attacks against DNS servers, though relatively rare until now, are considered especially critical because of their potential to bring down large numbers of Web sites.

In October 2002, all 13 of the Internet's root DNS servers were victims of a massive DDoS attack that raised concerns about the Internet's infrastructure but did little damage otherwise.

Just a week ago, VeriSign Inc. said that about 1,500 organizations worldwide had been attacked earlier this year by unknown hackers who employed botnets and DNS servers to launch a particularly devastating form of denial of service.