Customers of the ISP were without access for several hours on 4 October when a second switch card, which was supposed to provide resilience, failed while the failure of the first was being investigated. The problem knocked out the link between Demon's servers and the internet.
Gartner analyst Neil Rickard said, "Users should regard this as a warning - if the standards of reliability you need are high you should look at multiple networks.
"It shows the need for diversity of network provision. No provider is fully trustable, so decide whether you need two lines or satellite links or even two completely separate suppliers."
Demon technical staff got both devices back online after resetting the devices. The company is still carrying out a fault analysis on the cards.