Microsoft has admitted to causing a fault that made many of its Web sites inaccessible Tuesday.

The company said: "At 6:30 last night, a configuration change was made by a Microsoft technician to the routers on the edge of Microsoft's DNS (Domain Name Server) network.

"The mistaken configuration change limited communication between DNS servers on the Internet and Microsoft's DNS servers. This caused many of Microsoft's sites to be unreachable – although they were actually still operational – to a large number of customers throughout last night and today."

Translation DNS servers are responsible for translating addresses, such as www.microsoft.com or www.hotmail.com, into the numeric IP (Internet Protocol) addresses that computers use to communicate with each other. When the DNS system is not available or is corrupted, software in a user's computer is unable to find out the numeric address and so is unable to communicate with the desired server.

Microsoft said the fault was rectified at 5pm Pacific standard time when the original changes were removed - 22.5 hours after they were first uploaded. The company "immediately saw a massive improvement in the DNS network" when the settings were restored, it said.

Some of the Internet's most popular sites were affected, including the company's main Microsoft.com site, Web-based email site Hotmail.com, Web portal MSN.com, news site MSNBC.com, travel and vacation site Expedia.com, online car-sales service CarPoint.com and the Encarta.com reference site.