Problem with IP address

I'm using Windows XP (Prof) and recently bought a new hard drive. Previously, I was able to host multi-player games on a dedicated server (Gamespy) with no problems. I'm now getting a message that says I cannot host games because my internal IP address does not match my external IP address. I have noticed that my IP address changed after I switched modems (old one was bad). What do I do?? :

I assume you have cable? Normally that happens if you get a new MAC address.

Do you have any type of firewall on your PC or a router? I use gamespy and there's not an option to set anything like that.

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Yes, I do have a cable modem and no, I only have the fairwall that comes with XP and I disabled that. Any suggestions??

So no protection then?

That error can't be correct, because your internal IP can't be the same as your public IP.

Are you sure that's what it says exactly?

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