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Doc,
Unless your ISP will supply you with a static IP address (usually for a few extra bucks a month) I don't think what you are trying to do will work.

Unless you're paying for a static IP address from your ISP they are dynamically generating a new IP address for you each time one of your computers access there service (i.e usually when you restart the computer or access the net via there services) Your router is taking whatever IP address they send it and then again dynamically generating a unique IP address for each computer on your network.

I'm no IT person and the above example or explanation may not be completely correct but I'm guessing your running into trouble and your isp dynamically generating ip address for you is as good an explanation as any.