Understanding the purpose of network models

In the early days of networks, most systems utilized proprietary software and protocols that would only allow communication with other devices from the same manufacturer or those manufacturers that had access to those protocols. At the time, this was not much of an issue as organizations would purchase their equipment from the same manufacturer throughout, and there were no real means of communicating outside of your organization. However, over time, this has changed and there's now a need to communicate with systems owned by other organizations. Herein lay an issue.

It was unlikely that the other organization had equipment from the same manufacturer, so there was no way for these devices to talk ...

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