This lesson series will step through the OSI 7-Layer Networking Model

• Physical
• Data Link
• Network
• Transport
• Session
• Presentation
• Application

https://en.wikipedia.org/wiki/OSI_model

The data link layer builds on a functional physical layer and adds device unique IDs (MAC address) and controls access to the shared medium .. and detects and mitigates collisions. The data link layer is one of the most important layers, and creates the foundational networking layer which supports Layer 3 IP .. which is how the internet functions.