Skype

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To clear this up a little more, this can be a big problem. Years ago a single person (using a botnet?) was capable of at the single push of a button dc'ing my entire internet untill they felt like allowing me back on. Skype isn't the only way to get a person's IP, you can use PM/MASS messages too (tho I wont explain how).

I've seen people even go as far as to enter the default admin password into router and change password blocking all access. I had to push a button to reset the router.