In terms of blacklisting by businesses, Facebook was followed in OpenDNS's list by MySpace, blocked by 13 percent of subscribers, and YouTube, blocked by just under 12 percent.

The latter is undoubtedly on the list due to concerns about Internet bandwidth being wasted on movie downloads. Think what happens when a funny video e-mail gets forwarded around the office. Lots of people suddenly request a significant amount of data, which can bring a Net connection to its knees.

Facebook and MySpace are probably blocked by organizations that wish to stop employees wasting work time updating their status.

Interestingly, the top 10 blacklisted addresses also include ad.doubleclick.net (5.7 percent) and ad.yeldmanager.com (1.8 percent). Both addresses typically serve adverts and their blocking might indicate privacy concerns within organizations about . Alternatively, it might be that sympathetic network admins "switch off" adverts for their users.

Whitelisting allows OpenDNS users to let through sites that might otherwise be banned because the user has chosen to block entire categories of Websites.