Restricting the li being sorted within ul area

Question

I am using Knockout Sortable with template binding for sorting the li's within a ul. How can we restrict the li being sorted within a limited ul area? My code is allowing me to drag and drop any where in the page.

<div class="added-custom-field">
    <ul class="custom-fields-list" data-bind="sortable:{template:'control-template', data:CustomFields}">
    </ul>
</div>