"When users visit a website like Wired.com, the DNS system maps the domain name into an IP address such as 22.214.171.124. But if a particular site does not exist, the DNS server tells the browser that there's no such listing and a simple error message should be displayed.
But starting in August 2006, Earthlink instead intercepts that Non-Existent Domain (NXDOMAIN) response and sends the IP address of ad-partner Barefruit's server as the answer. When the browser visits that page, the user sees a list of suggestions for what site the user might have actually wanted, along with a search box and Yahoo ads.
The rub comes when a user is asking for a nonexistent subdomain of a real website, such as http://webmale.google.com, where the subdomain webmale doesn't exist (unlike, say, mail in mail.google.com). In this case, the Earthlink/Barefruit ads appear in the browser, while the title bar suggests that it's the official Google site.