Request.UserHostAddress

string VisitorIPAddress = Request.UserHostAddress.ToString();

public static String GetIPAddress()
{
    String ip = HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"];
    if (string.IsNullOrEmpty(ip))
        ip = HttpContext.Current.Request.ServerVariables["REMOTE_ADDR"];
    else
        ip = ip.Split(',')[0];

    return ip;
}

Request.UserHostAddress

Request.UserHostAddress