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Microsoft said late Wednesday that it was to blame for a fault that resulted in many of its Web sites being inaccessible to millions of users for almost 24 hours.

"At 6:30 [Tuesday] night, a configuration change was made by a Microsoft technician to the routers on the edge of Microsoft's DNS network," the company says in a statement.

"The mistaken configuration change limited communication between DNS servers on the Internet and Microsoft's DNS servers. This limited communication 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."

Domain Name Servers are responsible for translating addresses, such as www.microsoft.com or www.hotmail.com, into the numeric Internet protocol addresses that computers use to communicate with each other. When the DNS system is not available or 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.

"This was an operational error, and not the result of any issue with Microsoft or third-party products nor the security of our networks. Microsoft regrets any inconvenience caused to customers due to this issue," the company adds.

Microsoft says the fault was rectified at 5 p.m. PST when the original changes were removed -- some 22 1/2 hours after they were originally uploaded. The company "immediately saw a massive improvement in the DNS network," when the settings were restored, it says.

The problems hit a number of the Internet's most popular sites including the company's main Microsoft.com site, Web-based e-mail site Hotmail.com, Web portal MSN.com, news site MSNBC.com, travel and vacation site Expedia.com, online car sales and information service CarPoint.com, and the Encarta.com reference site.

As the business day progressed, users in parts of the U.S. gained access gradually to some of the software giant's Web sites. Some users on the East Coast reported no problems accessing any of Microsoft's sites Wednesday afternoon, while others in Chicago, Austin, Texas, and San Francisco reported difficulties.

"By mid-afternoon, I could get to Hotmail, MSN, and Expedia, but still had no luck with Microsoft.com," says Arturo Castellanos, a system administrator at Austin-based online travel company GeoPassage.

A user in London also says she was able to reach the e-mail and travel services but not Microsoft's home page, and most users in San Francisco were blocked from accessing any of Microsoft's properties late into Wednesday afternoon.

The problems also proved frustrating for the nearly 60 million Hotmail subscribers around the world, many of whom were cut off from their e-mail.

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