About three years ago I switched my whole family from Windows computers to Apple Macs. In fact, my eldest daughter got our first mac (a 15" Macbook Pro) as a gift for her sixteenth birthday. She is now nineteen and works behind the "Genuis Bar" at the local Apple store.
From time to time though, I get this notice on my iMac or Macbook Pro...
The solution (on OSX Mountain Lion) is as follows...
1. Close out of any applications that are connected to the Web.
2. Click on the Apple menu and choose System Preferences.
3. Select your active network interface from the pane on the left.
4. Click "Advanced" in the lower-right corner of the main pane.
5. Click the TCP/IP tab, and click the "Renew DHCP Lease" button.
The system will toss your existing IP Address and assign you a new one that doesn't conflict with others on your network.