Layer 1 - Data Link Layer

The data link layer is concerned with transmitting reliable data through the physical layer. As such, it defines such things as physical addressing (for example, the MAC address of a machine or device), network topology, line discipline, error notification, ordered delivery of frames, and flow control.

The IEEE has divided this layer into two sublayers: the MAC sublayer and the LLC sublayer. 

Continuing the rail road analogy, this layer is concerned with moving cargo onto and off of the railroad cars.