What you mean in the IP address that is seen from outside.
To get this IP address, you must use a service that provide this information manually or programatically (via API).
For example, to use 'http://whatismyip.com' the following code will obtain the IP programatically:
UTF8Encoding utf8 = new UTF8Encoding();
WebClient mywebClient = new WebClient();
String externalIp = utf8.GetString(mywebClient.DownloadData("http://whatismyip.com/automation/n09230945.asp"));
Response.Write("Your External IP Address is: " + externalIp );