Trials of an online ad system carried out by BT involving more than 30,000 of its customers were potentially illegal, says a leading digital rights lawyer.

BT has said it trialled a prototype of Phorm, which matches adverts to users' web habits, in 2006 and 2007.

The company did not inform customers that they were part of the trial.

Nicholas Bohm, of the Foundation for Information Policy Research, said tests without the knowledge of users were "an illegal intercept of users' data".

Uses

The most basic task of DNS is to translate hostnames to IP addresses. In very simple terms, it can be compared to a phone book. DNS also has other important uses.

Above all, DNS makes it possible to assign Internet names to organizations (or concerns they represent), independently of the physical routing hierarchy represented by the numerical IP address. Because of this, hyperlinks and Internet contact information can remain the same, whatever the current IP routing arrangements may be, and can take a human-readable form (such as "example.com"), which is easier to remember than the IP address 208.77.188.166. People take advantage of this when they recite meaningful URLs and e-mail addresses without caring how the machine will actually locate them.

The Domain Name System distributes the responsibility for assigning domain names and mapping them to IP networks by allowing an authoritative server for each domain to keep track of its own changes, avoiding the need for a central registrar to be continually consulted and updated.