The primary function of a network is to provide connectivity between devices. In the past, there were many different network protocols, each specific to certain devices or preferred for certain applications. Today, the majority of network communication is based on the Transmission Control Protocol/Internet Protocol (TCP/IP) suite. It is important to understand that the TCP/IP model is conceptually based on the Open Systems Interconnection (OSI) model, which is composed of seven layers. Each layer in the OSI model has a specific function, and a layer can be modified or changed without requiring changes to the layers above or below it.

1. Application Layer (Layer 7): This layer is responsible for providing the interface between the user’s software applications and the network. It enables communication between software applications and lower-level network services.
2. Presentation Layer (Layer 6): This layer is responsible for data translation, encryption, and compression. It ensures that data is presented in a format that can be understood by both the sender and receiver.
3. Session Layer (Layer 5): This layer manages and controls the communication sessions between devices. It establishes, maintains, and terminates connections.
4. Transport Layer (Layer 4): This layer is responsible for providing end-to-end communication between devices. It manages the flow of data, error checking, and recovery of lost data packets. The two main protocols in this layer are the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP).
5. Network Layer (Layer 3): This layer is responsible for routing and forwarding data packets between devices on different networks. The main protocol used in this layer is the Internet Protocol (IP).
6. Data Link Layer (Layer 2): This layer is responsible for providing reliable data transfer between devices on the same network. It handles error detection and correction, as well as the framing and encoding of data. The two main sub-layers in this layer are the Logical Link Control (LLC) and the Media Access Control (MAC).
7. Physical Layer (Layer 1): This layer is responsible for transmitting the raw data bits over the physical medium, such as copper wire, fiber optic cable, or wireless radio signals. It deals with the electrical and mechanical aspects of data transmission.

The seven-layer OSI model provides a conceptual framework for understanding how different protocols and technologies work together to facilitate network communication. By separating the functions into distinct layers, it allows for greater flexibility and interoperability between different devices and systems.