请问，为什么我的每一个注册用户的ip地址都显示是0.0.0.0

为什么我的每一个注册用户的ip地址都显示是0.0.0.0，可以确定代码是没错的

<?php

if(trim($_POST['password'])!=trim($_POST['firmPassword'])){

   exit("两次输入的密码不一致");

}

$userName=trim($_POST['userName']);

$password=md5(trim($_POST['password']));

$time=time();

$ip=ip2long($_SERVER['REMOTE_ADDR']);

echo $ip;

$conn=mysqli_connect("localhost","root","");

if(mysqli_errno($conn)){

   echo mysqli_error($conn);

   exit;

}

mysqli_set_charset($conn, 'utf8');

mysqli_select_db($conn,'zhuce_php');

$sql = "insert into user(`userName`,`password`,`createtime`,`createip`) values ('" 

. $userName . "','" . $password . "','" . $time . "','" . $ip . "')";

$result=mysqli_query($conn,$sql);

echo '当前用户插入的ID为:'.$conn->insert_id;  

if($result){

  echo "成功<br/>";

}else{

  echo"失败";

}

mysqli_close($conn);

?>