Proxy servers

As a general rule, any computer that is connected to the internet can be located because it has a unique identity, known as an IP address. The name and address of the person to whom the IP address is registered will, in turn, be known to their internet service provider (ISP). Sometimes, however, a computer owner can connect to the internet via a series of computers known as 'proxy servers' which obscure the owner's location. Only with extremely complicated - and time-consuming - investigation can the address be traced.

Botnets

Often, offensive content does not sit on the extremist's own computer but on another machine whose owner is unaware that their system has been hijacked. These so-called 'zombie' computers are controlled by the extremists, who attack the victim's computer from outside with code known as 'malware' which allows them to take control of the machine. The attacks take place from behind the walls of so-called 'bullet-proof hosting' sites which make tracing difficult.

'Dynamic' web addresses

Savvy website owners now have the ability to change the location of sites on which they post content on an almost constant basis - in some cases as often as once a second. Using a technique called fast-flux DNS, they can change the server which a person visits in order to view a page, often between countries. The visitor still types the same domain name, but the shifting servers make tracing the site difficult.

Peer-to-peer networks

Much of the search for online extremists focuses on websites, but there are a range of other methods that can be used to communicate via the internet, including 'chat' and peer-to-peer technologies, which can be more difficult to track. In a peer-to-peer network, each member acts as a 'router' - or hub - for all the others, meaning that activities can be easier to conceal than if everyone is visiting a single site.

Legal constraints

If a site with offensive content is hosted on an ISP in another country, the ability of police to compel the ISP to reveal details of the site’s users depends on the laws of that country. Generally the ISP hosting the content can be established via the organisation which has registered the site, known as an internet registry, which is separate to the ISP. (Sites which end with .co.uk have to be registered with Nominet, a British company, for instance.) Many countries now work with the UK to trace online extremists, but others - including China, Russia, Korea, and Brasil, can be less co-operative, experts say.

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