-To end this sequence, here is a "Going Further" episode on the Internet Protocol and the hourglass. Why should we be interested in this hourglass-shaped model? It is a way to emphasize the current essential role of Internet Protocol in networks. It will also allow me to introduce the layer model notion, that we have not seen yet, to be seen during Internet access. If we look at interconnection, a term you know since I mentioned it, between different technologies such as fiber, ADSL, a geostationary satellite and Wi-Fi. If we want to interconnect all this, we need to design specific devices between each technology, a device between Wi-Fi and fiber for example, a device between fiber and ADSL, between ADSL and GEO, and so on for every technology you have. Each new technology introduces new devices, thus new costs. Any older technology will need to interconnect with the new. If you want to interconnect three technologies, you will need to design a specific device. All this has a cost, is specific and quickly becomes obsolete. Whereas if we use a common protocol to talk between all this, IP, our problems are solved. All technologies can therefore be directly interconnected. How is it possible ? IP has become a universal protocol that does not require much from the underlying technologies, a priori. We are starting to see the beginning of this model, the lower triangle. Let us now see the upper part. Just like each technology wanting to talk with another, requiring a device be implemented, if you have an application, voice for example, and only technologies, then you will have to create your application for each technology, thus as many different applications. A voice for Wi-Fi application, for fiber, ADSL, or geostationary. In that case, if you have another application, for example a twit, you will need to write this application for each technology while making sure that these applications can potentially work together to allow someone on the fiber to talk with someone on the geostationary satellite. It is quite complicated. Likewise, it would be much smarter to have a common protocol, IP, to allow all technologies to talk together for the same application. If you use IP as an interface, no matter the technology behind, your voice operates on IP, technologies operate on IP, so there is no issue. The interconnection, interoperability and integration protocol is IP. This gives us the hourglass model with a variety of applications and of technologies, and at the heart of the model, IP, a very strong core that might explain why, today, it is quite difficult to change this protocol in the global network. Because if you change it, how can the transition be handled so that applications can communicate no matter the technologies used? This also introduces the beginning of a layer model represented by three rough layers. The application layer first, with all the various existing applications, then the physical technology layer, for interconnection and binding, and in the middle, the network. This network layer is IP. We will get back to this model in more detail when we study Internet.