Access ASP.NET application from internal network

hi,

We are currently developing a business ASP.NET application that you can only access from a public IP address (ie the internet) even if we are on the same network of portal web server.

The problem is that we want to access to the web server does not come only from the internet but:

1 If access to the portal is inside the corporate network it is accessed by the internal corporate network and no internet. 
2 If you have access from the external network that is Internet access

How can I configure IIS to such a configuration ??

Thank you in advance.