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Some users were without access for three days. Ian Hood, director of communications for Thus, which owns Demon said: "A set of unusual circumstances have combined to cause this problem."
Internet connections from Telehouse in New York, a major site for transferring information across the Internet has been badly affected by the attack on the World Trade Centre. This meant that Demon, like other ISPs, had to look for other sources of bandwidth to keep its services running.
"We needed to reconfigure our network to take into account the loss of services at Telehouse," said Hood. "During this complex procedure, something went wrong.
"Around the same time, the first wave of the Nimda virus hit and although our infrastructure wasn't affected, our customers who were infected by the virus started generating huge amounts of traffic which flooded our servers and gateways." Hood added.
Hood dismissed reports that all of Demon's customers had been affected but admitted that less than 10% may have intermittent failure.
Several Demon customers claimed the ISP was unhelpful in responding to queries. Hood responded: "We were getting calls from both users infected by the virus who assumed that the problem was down to us. This is not a situation that we could have predicted and we were not able to answer every call as we normally would. We were stretched but we fixed the problem as quickly as we could."
Hood believes that the Nimda virus affected other ISPs just as severely.
"The situation is under control now and our customer helpdesk service is running as normal. We will be meeting on Friday to look at how we can better protect ourselves and our customers from this kind of thing in the future," he added.
Cw360 readers confirmed that normal service had been resumed and many had been able to contact Demon's helpdesk to gain more information.