Flashback : Understanding the Internet Reference Model

Network Access Layer
The lowest layer. Contains protocols that the computer uses to deliver data to the others computer and devices that are attached to the network. The protocols at this layer perform three distinct function:

1. Define how to use the network to transmit a frame, which is the data unit passed across the physical connection
2. Exchange data between computer and physical network
3. Deliver data between two devices on the same network. To deliver data on the local network, the network access layer use the physical address of the nodes on the network.

Internetwork Layer
Responsible for routing messages through internetwork
Two types of device are responsible:

1. Gateway - Computer that has two network adapter card. This computer accepts network packet from one network on one network card and routes those packet to a different network via the second network adapter card.
2. Router - A dedicated hardware device that passes packets from one network to a different network. 

Host-to-Host Transport Layer
Responsible for providing end-to-end data integrity and provides a high reliable communication service for entities that want to carry out an extended two way conversation.

Application Layer
The top layer. Provide functions for users or their programs and it is highly specific to the application being perform. It provides the service that user applications use to communicate over the network. It is the layer in which user-access network processes reside.

The layer includes all applications protocols that use the host-to-host transport protocols to deliver data. Others function that process user data such as data encryption and decryption, compression and decompression

The application layer also manages the sessions (connections ) between cooperating applications. TCP/IP uses the term "socket" and "port " to describe  the path ("virtual circuit") over which cooperating applications communicate.

For cooperating applications to be able exchange data, they must agree about how data is represented. The application layer is responsible for standardizing the presentation of data.