Microsoft has revealed that it has been the target of a "denial of service" attack against the routers that direct traffic to its Web sites.

The attack knocked Microsoft.com, MSN.com and other Microsoft sites off the Internet for the second time this week. The company caused similar problems for itself this week when an incorrect server-configuration change disabled its name servers.

The denial of service attack lasted for four hours.

A denial-of-service attack overwhelms a router or Web site with requests, preventing normal communications and shutting down the router or Web site.

Router ID The attacked router was inside Microsoft, but the source could not confirm if it was the same router that went down after Tuesday's problem.

The source said Microsoft is taking a good, hard look at its infrastructure and will evaluate if anything needs to change.

"They have opened themselves up to these sorts of attacks by letting the world know they have a single point of failure," said one network administrator who asked not to be identified.

The attacks came nearly a year after a hacker calling himself MafiaBoy crippled eight major Web sites, including CNN.com, eBay.com and Yahoo.com.