右边的广告用了fixed定位还是没有起到效果，为什么？

来源：4-3 编程练习

weibo_淡写丶甜甜的回溢_0

2018-01-06 19:34:30

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
<title></title>
<style type="text/css">
      *{padding:0;margin:0;}
      .div{
            width:100%;
            height:4100px;
            background:url(http://climg.mukewang.com/59c9f7ce0001839219034033.png) center top no-repeat;}
      /*左侧广告*/
      .div1{
            width:500px;
            height:250px;
            top:0;
            left:0;
            bottom:0;
            margin:auto 0;
            position:fixed;}
      /*右侧广告*/
      .div2{
            width:500px;
            height:250px;
            top:0;
            right:0;
            bottom:0;
            margin:auto 0;
            position:fixed;}
</style>
</head>
<body>
<div class="div">
    <div class="div1">
      <img src="http://climg.mukewang.com/5a3383d00001a3dd02240364.png"/>
    </div>
    <div class="div2">
      <img src="http://climg.mukewang.com/5a3383c70001f1b702240364.png"/>
    </div>
</div>
</body>
</html>