Accessing external server site in case user is not on VPN in Asp.Net C#

I have a situation. Our company has two server one is accessed from VPN like intranet etc. and another is external server for external users i.e. non employee. Now what i want to do is I need to create one page in asp.net to capture some information. So what it should do is if the user is on VPN then he should be redirected to our internal server and if the user is not on VPN it should be redirected to external server.

Any pointers would be appreciated.