Conceptual animation showing internet traffic, with normal data traffic white and malicious traffic red. Occasionally a connected server may be taken down by a distributed denial-of-service attack, where a large number of typically virus-infected and remotely-controlled computers direct a huge amount of web traffic to a single site, overloading its servers and causing it to cease to function online. The animation focusses on such an attack directed at servers in London, UK, which has left large parts of a network unavailable.