The document discusses network models and layers. It describes a layered model with physical, data link, network, transport, and application layers. The physical layer transmits individual bits between nodes. The data link layer transmits frames between nodes and uses physical addresses. The network layer delivers packets from source to destination, using network addresses. The transport layer delivers messages between processes, using port addresses. The application layer provides services to users. Figures are included to illustrate examples of how data is encapsulated and transmitted across each layer.