GoDaddy, the internet's largest domain registrar, was hacked on Monday morning by a member of the Anonymous hacking group. Although it was not clear how many websites were affected, "DNS" or Domain Name System, "GoDaddy" and "Anonymous" are trending on Twitter, as thousands of clients and observers comment on the outage.

A hacker identifying himself as the Security leader of Anonymous claimed responsibility shortly after the attack. @AnonymousOwn3r said he hacked GoDaddy because: "I'd like to test how the cyber security is safe and for more reasons that I can not talk now."

Also on Twitter, GoDaddy customer service representatives said they were overwhelmed with complaints, but were working on the problem.

GoDaddy is also one of the biggest hosting providers on the internet, housing the content for millions of websites. Domains affected by the attack range from small community groups like Little Rock Mommies, an online group for mothers in Central Arkansas, to Coursera, an online education company, to JHill's Staffing Services, a Los Angeles-based recruitment & career consulting firm.

Anonymous is a loose collection of hackers who co-ordinate online and infiltrate websites as a form of protest against governments or corporations they view as oppressive or corrupt. In a tweet, @AnonymousOwn3r claimed that he acted alone and without the cooperation of other Anonymous members.

Since its creation in 2003, Anonymous has claimed responsibility for several high-profile hackings including the recent takedowns of several UK government sites in April 2012 and that of CIA.gov in February 2012.