I am taking a course preparing me for the Network+ certification. I wanted to know what the purpose of a private IP is when a MAC address exists. From what I understand, the MAC address is used to forward data in a segment of a network using the Data Link layer. It looks like a private IP does the same thing except at the Network layer. Does a private IP have an advantage compared to just using the MAC address? Maybe if a network has multiple segments? Maybe if a router connects to two switches or something?
I also have another question regarding the OSI model. The course describes same layer interaction and adjacent layer interaction. I wanted to know when same layer interaction ever takes place. Based on the explanation, I think that adjacent layer interaction is the only interaction that can happen in the OSI model. That's because it is explained that adjacent layer interaction happens when a layer uses functions from a layer or provides functions to a layer. However, every single layer interacts with each other. When and how does same layer interaction occur?