This should work:
//this gets the ip address of the server pc
public string GetIPAddress()
{
IPHostEntry ipHostInfo = Dns.GetHostEntry(Dns.GetHostName()); // `Dns.Resolve()` method is deprecated.
IPAddress ipAddress = ipHostInfo.AddressList[0];
return ipAddress.ToString();
}
http://wec-library.blogspot.com/2008/03/gets-ip-address-of-server-pc-using-c.html
OR
//while this gets the ip address of the visitor making the call
HttpContext.Current.Request.UserHostAddress;
http://www.geekpedia.com/KB32_How-do-I-get-the-visitors-IP-address.html