Years ago the way to connect two devices together was more troublesome than it is in the modern day. There was even a time when two devices that were not made by the same company were not able to communicate between each other.
Large corporations like IBM and others created their own networks which were great at communicating with their own proprietary components but could not communicate with other devices other than their own.
In 1969 ARPANET was created by the DOD along with Packet Switching which changed the way computers talked to each other.
There were two major networking models that were invented. The TCP/IP and OSI (Open Systems Interconnect) model. Today every computer supports and uses TCP/IP to communicate to other devices. OSI is very similar but they add the Presentation and Session layers which are still useful in understanding the data layer but because of the way TCP/IP was adopted to nearly every device, we can commonly refer to TCP/IP instead of OSI. Very confusing, but for the CCNA it’s great to know both and understand how and why each model is used.
The OSI and TCP/IP Layers Explained