Dynamic Adaptive Streaming over HTTP (DASH) is a new and promising streaming protocol, based on the Media Presentation Description (MPD) specification. With the increasing demand for Internet video streaming, methods for profiting from video services are gaining increased interest. In this paper, we propose a novel algorithm for server side video ad insertion over the DASH standard. The proposed method is compatible with DASH and does not require any modifications at the client side, such as dedicated players, or any modification in the MPD definitions. Furthermore, the algorithm enables the client to always receive the advertisements regardless of his player software. Our novel approach considers the MPD URLs as encrypted URLs for pointer mapping. This will allow for deciding in real time whether a specific URL will point to an ad or to the original video stream segment. Therefore, the solution enables us to consider VOD ad insertion as similar to live ad insertion. In a comparison between the DASH video streaming server with ad insertion, as define in the standard, and our ad insertion solution, the results showed that our solution provide a dynamic ad insertion system while only slightly increasing the CPU load. As far as we know, this is the first DASH server side ad insertion solution for the ISO Base Media File Format (MPEG-4 part 12) container.