Apparently you're using an old version of LimeWire. You can try uninstalling it and reinstalling the current one (1.8b as of this writing). You can see your machine's IP address in the TCP/IP control panel. Not all IPs starting with 172 are private--only those whose second number is between 16 and 31. If you have such a private IP, use Force Local IP; otherwise don't. The Force Local IP address must be your public IP address. If you don't know it, go to http://www.dslreports.com/scan and the Java applet will tell you. You may also try Mactella; the current version (1.3.4) is quite decent.
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All of the above assumes you're using OS9. Since client installations under OS X are far less controlled, it may require other considerations.