No network book seems to be complete without first going over the Open System Interconnection (OSI) model. The model is a conceptional model that componentizes the telecommunication functions into different layers. The model defines seven layers, and each layer sits independently on top of another one, as long as they follow defined structures and characteristics. For example, the network layer, such as IP, can sit on top of different type of data link layer such as the Ethernet or frame relay. The OSI reference model is a good way to normalize different and diverse technologies into a set of common language that people can agree on. This greatly reduces the scope for parties working on the individual layers and allows them to go in depth on the specific tasks without worrying about compatibility:

OSI Model (source: https://en.wikipedia.org/wiki/OSI_model)

The OSI model was initially worked on in the late 1970's and later on published jointly by the International Organization for Standardization (ISO) and the now called Telecommunication StandardizationSector of the International Telecommunication Union (ITU-T). It is widely accepted and commonly referred to when introducing a new topic in telecommunication.

Around the same time period of the OSI model development, the internet was taking shape. The reference which model they used is often referred to as the TCP/IP model since the Transmission Control Protocol (TCP) and Internet Protocol (IP), because originally, this was what the protocol suites contained. It is somewhat similar to the OSI model in the sense that they divide the end-to-end data communication into abstraction layers. What is different is the model combined layers 5 to 7 in the OSI model into the Application layer while the Physical and Data link layers are combined into the Link layer:

Internet Protocol Suite (source: https://en.wikipedia.org/wiki/Internet_protocol_suite)

Both the OSI and TCP/IP models are useful in providing a standard for providing an end-to-end data communication. However, for the most part, we will refer to the TCP/IP model more since that is what the internet was built on. We will specify the OSI model when needed, such as when we are discussing the web framework in the upcoming chapters.