The following link outlines types of network attacks that may occur.

A white paper on network security.

Hardly a week goes by when some new security hole isn't announced in Windows, IIS, Exchange, etc. In fact, the security research firm Relevant Technologies (www.relevanttechnologies.com) reports that there are an average of ten new security vulnerabilities discovered each week. All of these holes make it pretty tough to keep up with all of the latest security patches, especially if your network has never been attacked and you don't really have a strong incentive to keep up with the patches. The sad truth is that the vast majority of network attacks go unnoticed. In fact, your network could be under attack right now and you might not even know it.

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Describes different types of network attacks. From Lycos.com Asia.