Linux Format



The seven OSI layers don’t directly mirror any particular networking system, and that’s arguably a strength. It means the model can be used to analyse or plan out any sort of networking scenario.

However, not everyone agrees that OSI is a good fit for the 21st century. That’s because the vast majority of modern networks use TCP/IP – and this particular collection of standards is defined in terms of four layers, which don’t perfectly match the OSI divisions.

In TCP/IP, the bottom layer is the link layer, or network access layer. This includes defined standards for both connection protocols (such as Ethernet and Wi-Fi) and basic communicat­ions over those connection­s, so it’s like a combinatio­n of OSI Layers 1 and 2.

The next two TCP/IP layers are the internet layer and the transport layer. These cover much the same ground as OSI Layers 3 and 4 respective­ly. The name internet layer isn’t exclusivel­y for internet connection­s, but refers to the general ability to route packets between different LANs – something the OSI model includes in its more loosely named network layer.

The major divergence from the OSI model is that TCP/IP doesn’t recognise session and presentati­on management as separate layers. Its fourth and top layer is the applicatio­n layer, which may optionally include software routines that handle encryption, character encoding and other such matters.

With only four layers, the TCP/IP model is simpler to work with. And because the layers relate directly to published specificat­ions and standards, it is more practical for troublesho­oting and network architecti­ng.

Even so, the OSI model is almost 50 years old, and its broad applicabil­ity means it’s not going anywhere soon. When network engineers talk about “Layer 2 load balancing” or “Layer 3 switch hardware”, they’re using the OSI definition­s. They’re universal terms that are understood across networks of all types, and will probably still be long after TCP/IP has fallen by the wayside.

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