codice:
#divgrande {
 width: 400px;
 height: 400px;
 position: relative;
 border: 1px dashed #dddddd;
}
#divpiccolo {
 border: 1px dashed #dddddd;
 position: absolute;
 right: 0px;
 top: 0px;
 width: 50px;
 height: 50px;
}
<div id="divgrande">
<div id="divpiccolo">
</div>
</div>