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The backstory of the OSI model and Ethernet seems to have little relevance to “modern” networking. But upon closer inspection, many of the fundamentals of networking haven't changed in 50 years. Essentially, the goal of networking is and always has been to enable applications to transfer data. The challenge of networking is to make that happen without having to hard-code gritty networking details into each application. That's where the layering concept of the OSI model comes in. Each layer is an abstraction that conceals some part of the network from the application. Although the OSI model is abstract, the protocols that networks implement today in a mostly consistent manner are very real.

Data Link layer protocols such as Ethernet, PPP, and WLAN conceal the details of network interfaces and provide a common frame format that can be used across different media types. Network layer protocols such as IPv4 and IPv6 hide the different Data Link layer protocols in use across different networks. This is why the Network layer is sometimes called the Internetwork layer. Applications can use Transport layer protocols such as TCP and UDP to distinguish different communication streams.

The upper layers—Application, Session, and Presentation—have always posed a challenge for the OSI model. It's clear that these are all one layer: the Application layer. What makes Application layer protocols unique is that they are “the end of the line.” That is, there is no higher-layer protocol for an application to pass data up to. Hence, many things that we previously thought of as simply networking protocols are actually applications: ARP, BGP, EIGRP, and OSPF, just to name a few. PDUs generated by these aren't passed up to any higher-layer protocol. When you look at the Application layer in this way, the network suddenly looks a lot simpler.