TCP/IP model
The four-layer model of the internet protocol suite (link, internet, transport and application) that describes how working networks are built and maps onto the seven OSI layers.
The TCP/IP model describes the internet protocol suite in four layers, as set out in RFC 1122 (1989): the link layer, the internet layer, the transport layer and the application layer. Study sources commonly call the lowest layer network access, and the model is also known as the DoD model, after the research that produced it. Where the OSI model was designed as a reference framework, TCP/IP describes protocols already in use on the internet.
The layers map onto OSI approximately. The application layer covers OSI layers 5, 6 and 7, so session and presentation functions sit inside the applications. The transport layer matches OSI layer 4, home of TCP and UDP. The internet layer matches OSI layer 3, home of the Internet Protocol and ICMP. The link layer covers OSI layers 1 and 2. Some texts split that bottom layer and show five layers. As data moves down the stack, each layer wraps the one above it through encapsulation.
Exam relevance: a scenario may name a protocol or a device and ask where it operates, and the answer depends on which model the question uses. Candidates are expected to translate between the four TCP/IP layers and the seven OSI layers, and to recognise which vocabulary a question is using.