custom validation rule for IP in form

I am very new to codeigniter(have over 10 years php procedural experiance) and seems like i have run into issues from start. I am creating a auth for my site and in the sign up for i want to check IP not only if its valid but against a custom validation rule i made. I get the IP in the controller via "request->getIPAddress()".

Is there a way i can add the IP in the post method so i can use validation rules for it like they are used for fields that are passed from the form in my view?