You have an internal and external IP. That website will give you your external IP. You need to forward ports to your internal IP and if you haven't made it static, you will have to reenter that IP every time you connect to the router (or it could get the same IP anyways depending on your config). If you map your MAC address to an IP you can always have the same IP when reconnecting and therefore not have to reset your ports. Static IP settings are normally in advanced settings. To get your MAC address, right click on my computer and click properties, I think it is somewhere in those menus, maybe not.