string requestClientIpAddress = HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"];
if (string.IsNullOrEmpty(requestClientIpAddress))
requestClientIpAddress = HttpContext.Current.Request.ServerVariables["REMOTE_ADDR"];
if (string.IsNullOrEmpty(requestClientIpAddress))
requestClientIpAddress = HttpContext.Current.Request.UserHostAddress;
经过测试 存在负载均衡的时候 ,HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"] 取出的是真实的客户端 IP地址 ,而HttpContext.Current.Request.ServerVariables["REMOTE_ADDR"] 和 HttpContext.Current.Request.UserHostAddress 取出的是被分配的保留地址
原文：http://www.cnblogs.com/LJQ-Jason/p/6475991.html