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PeerGuardian contains a list of "unsafe" IP addresses to which it denies all connections, that is, unsafe for people engaging in various forms of copyright infringement. This includes a rather paranoid collection of companies involved in various industries, those involved in the fight against piracy, and a great number of other addresses blocked based on sheer conjecture.

And it doesn't work. Even assuming the blacklist were comprehensive and accurate (far from it), blocking direct connections with certain IP addresses will not stop them from gaining access to yours. What it will do is prevent you from accessing certain legitimate websites and slow peer-to-peer connections.
 
To add to that comment -- LittleSnitch is an app you can purchase that's perfect for the paranoid that will warn you about any outgoing connections from your Mac to the internet, and lets you chose whether you want to allow or deny the connection just once or forever.
 
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