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Layered internet architecture > Hourglass architecture - Pg. 284

284 APPENDIX The layered Internet architecture and network protocols In this Appendix, we discuss more details on the operation and protocols of the link layer, network layer, and transport layer. The application layer is discussed in more detail in Chapter 9. Hourglass architecture An important aspect of this network architecture is that there is a single network layer protocol. Internet Protocol version 4 (IPv4) is the only network protocol used in the current Internet [with the exception of Internet Protocol version 6 (IPV6), which is being deployed incrementally]. This "hourglass architecture" is shown in Figure A-1, which shows the five protocol layers of the Internet and a selection of protocols for each layer. In the network layer, there is only one protocol--the Internet Protocol (IP). The restriction to IP at the network layer and the diversity of protocols in other layers give the hourglass architecture its "shape." The restriction of using a single network protocol is necessary to ensure interop- erability between all network systems in the network. With the layered protocol stack, however, it is possible to support diversity at layers above and below the net- work layer. For example, numerous different link layer protocols are in use to pro- vide point-to-point connectivity between neighboring systems. Because the link