IIS URL redirect

I want to enable access to the same website from two different URLs. 
The website is hosted for internal use. 
The internal URL is internal.example.com

But now I want to open this website to the public using external.example.com

There is no option to add a new building to this website. 

How can I enable URL redirect on IIS to achieve this?
I want to access all the internal website pages from an outside URL without any issues. Is there an option to achieve this on IIS?