System.Web.HttpContext.Current.Request.UserHostAddress;

I am using the following code to obtain the user I.P. address but something strange is happening. I am, receiving the same ip address every time no matter if I am on my desktop or on my iPhone. I am on our network where the web servers live when I hit the page from my desktop but on my iPhone I was not even in the building. I have not run into this issue before, the ip address that I am getting back is an internal one assigned to the web servers.

string ip = System.Web.HttpContext.Current.Request.UserHostAddress;
Response.Write(ip);

Outside of my network I still get the same ip address 172.16.0.22 which is a router address, not our external ip. Is there a way to obtain the external ip address.