The protocol stack used on the Internet is the Internet Protocol Suite. It is usually called TCP/IP after two of its most prominent protocols, but there are other protocols as well. The TCP/IP model is based on a fi ve-layer model for networking. From bottom (the link) to top (the user application), these are the physical, data link, network, transport, and application layers. Not all layers are completely defi ned by the model, so these layers are “fi lled in” by external standards and protocols. The layers have names but no numbers, and although sometimes people speak of “Layer 2” or “Layer 3,” these are not TCP/IP terms. Terms like these are actually from the OSI Reference Model. The TCP/IP stack is open, which means that there are no “secrets” as to how it works. (There are “open systems” too, but with TCP/IP, the systems do not have to be “open” and often are not.) Two compatible end-system applications can communicate regardless of their underlying architectures, although the connections between layers are not defi ned.
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