Intrusion Prevention System technology significantly increases the level of protection that Symantec Endpoint Security gives to your network. You should always have IPS enabled on your network
Intrusion Prevention System technology is strong, effective technology that prevents malicious files from getting to your hard drive in the first place
Unlike antivirus, which looks for known malicious files, IPS scans the network traffic stream in order to find threats using known exploits and attack vectors. IPS does not detect specific files, but rather specific methods that can be used to get malicious files onto your network. This allows IPS to protect against both known and unknown threats, even before antivirus signatures can be created for them.
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http://www.symantec.com/business/support/index?page=content&id=TECH95347