Anyway I don’t think I’ve seen a single post relating to the OSI model being referenced which is probably the most complete definition of internetworking.
The OSI model is also
not the definition of "The Internet" nor is it the definition of "internetworking". The OSI model is the model of ALL computer networking and it describes what networking actually is inside any hardware that can network.
Tha physical layer is most known by people as the Ethernet port and the patch cables. In Wifi, it would be like BNC connectors and antennas
Datalink layer is the layer that we can observe when we see the connection light on those ethernet jacks. In Wifi, it's most often displayed with software - some indication that you're physically connected to the network which would only happen if you accurately provided the security credentials. Having layers 1 and 2 working does not mean you can send and receive packets yet from your software...
The Newtork Layer would be best understood as that function within the hardware the provides the means of maintaining a connection from one node to another (one ethernet port to the ethernet port that it is directly connected to) - in Wifi this is virtualized, and it would include the MAC address of the port in that function.
Tht Tansport layer is the function within the hardware the defines the structure of the packets that traverse between nodes. It would also include error correction methods etc.
The session layer now begins to reach up beyond the hardware and is a function of the operating system at a low level. It is defined within the protocols being used. TCP/IP and UDP for example would have distinct and different ways of implementing the session layer.
Then there are the presentation and application layers, the presentation layer being the layer that brings packets to your applications like Chrome etc. and the Application layer being your apps themselves.
The OSI model describes what is happening inside a computer, or a router, or a switch or any other device that can operate on a network.
THE INTERNET happens in layer 3 mostly because layer 3 is where ROUTING between different networks happen.
But by no means, is the OSI model a definition of the Internet. The Internet exists within the context of the OSI model as does ANY network.
But the Internet is literally defined as the Inter-connection of otherwise disconnected, independent networks.
So if say you have three companies and they each exist in three different buildings and those buildings are geographically dispersed either throughout a city where their distance apart is such that they don't exist on the same property. When you CONNECT those three buildings so that any of the buildings can talk to any of the other buildings, you have now created an INTER-NETWORK ( a network of networks) and "THE INTERNET" is the label given to the Inter-network that exists that connects countless other networks which exist all over the planet. And we access the Internet via layer 3 of the OSI model which is provided to us by an Internet Service Provide by some means such as an Ethernet handoff or a fiber handoff maybe through a multiplexer, or a modulator of some kind ... cable modems etc. Some ISPs sell Internet through WiFi access points over large distances often called terrestrial networks or "last mile" networks, and there is also Satellite connections into the Internet, even point-to-point laser is one means of offering a connection into an internetwork or the Internet ...
But don't be confused, the OSI model is NOT a definition of "The Internet" just as TCP is also not a definition of "The Internet" ... however, the Internet exists within the OSI model and it does implement TCP as one protocol that computers use to communicate with each other on the Internet. UDP is also used often and operates independently of TCP and is a much more efficient protocol for things like video broadcasting and even large data transfers when it isn't necessary to use the heavy error correction that is built into TCP (making UDP much faster because of lower overhead in the protocol).
Other protocols such as FTP and SFTP and HTTP actually operate on top of TCP and they rely on TCP for packet delivery. They simply "add" to TCP: for example, FTP requires a connection stream to be established after a handshaking process has transpired with success and that process requires a username and password, where as HTTP and HTTPS are just overlays on TCP that are intended to transfer hypertext, HTTPS adding the requirement that the hypertext be encrypted and signed by a trusted certificate authority etc. Those would be presentation layer functions when thinking in terms of OSI model, and there are a lot of them out there... some of the more frequently used presentation layer protocols would include SSH, SFTP, Telnet etc.
But remember ... all of those protocols can be used within the confines of a single computer and do NOT require a computer to be connected to an actual network for them to be used. For example, if you installed a web server on your computer, then used Google Chrome on that same computer to pull up a web page that your computers webserver was offering, you could view those web pages locally without ever being connected to a network, and all seven layers of the OSI model would be satisfied within the context of just that one computer that is NOT on a local network nor is it on the Internet because OSI describes ALL networking, whether its local or wide area, or campus networks, or metropolitan networks, or global networks, the OSI model literally exists within each and every device that is capable of communicating on a network.
Examples of hardware that DO NOT exist within the OSI model would be ...VCRs ... or DVD players that don't have Ethernet ports on them ... or pretty much any TV set before the "Smart TV" ... film projectors, film-based cameras ... any analog telephone that must be connected to a POTs phone line ... if it doesn't have WiFi nor an Ethernet port on it, then it's not part of the OSI model.
If it DOES have an Ethernet port on it, or a WiFi radio in it, then technically it also contains all seven layers of the OSI model whether or not it's connected to The Internet.
The OSI model is just a model ... it's a description of the functions that must exist within a device before that device will be able to communicate on a network.
99% of my need of the OSI model is specifically when I am troubleshooting a problem on a network. The OSI model can be invaluable when you need to understand where a problem exists so that you know how to approach it and fix it.
A good analogy would be to say that in order for human beings to communicate with each other, they would need a body, a brain, and a mechanism from which to transcribe the thoughts in the brain to a tangible representation that could be then interpreted by other human beings who also have a body and a brain, but the body would be layer 1, vocal chords and hands would be like layer 2 (the post office could be seen as layer 3) ... the eyeballs or the ears could be the transport and session layers ... up to the comprehension of data being the application layer ...
again ... its not a model that is unique for the Internet NOR internetworks, nor does it have any specific connection to the Internet other than the Internet existing within layer 3 pretty much exclusively. OSI applies to all digital hardware that is network capable.
Mike