Which OSI layers are encompassed by the TCP/IP model's Application layer?

• A. Application, Presentation, and Session
• B. Application and Transport
• C. Physical and Data Link
• D. Network and Transport

Answer: (A)

In TCP/IP, the Application layer handles the functions of the upper OSI layers—those that deal with end-user applications, data formatting, and session management. Specifically, OSI layers five (Session), six (Presentation), and seven (Application) are encompassed because the protocols and services the Application layer supports (like HTTP, DNS, and FTP) rely on dialogue control, data representation, and application-level processing that the OSI model assigns to those top layers. The TCP/IP Application layer therefore consolidates those three OSI layers into one, since the upper-layer concerns are implemented together in practice.

Lower OSI layers map to other TCP/IP layers: Physical and Data Link correspond to the Link/Network Interface layer, Network maps to the Internet layer, and Transport remains as a separate layer within TCP/IP. That’s why only the upper three OSI layers fit into the TCP/IP Application layer.