Need help portforwarding tekkit server with right IP.

When I try and post my tekkit server on planet minecraft it says I am using a local IP and it can't accept that. It then shows me a website to find the right IP, I click on the website and see my IP, but when I try to portforward my tekkit server on that IP, my router's website says my IP is a invalid IP.
Could anyone tell me someway I could get the right IP to work.