Which OSI layer is not part of TCP/IP model?

The OSI (Open Systems Interconnection) model is a conceptual framework with seven layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application. In contrast, the TCP/IP model is a more streamlined architecture used for the actual implementation of the internet.

In the TCP/IP model (whether the original 4-layer or the updated 5-layer version), the functions of the OSI Session layer and Presentation layer are merged into a single Application layer. Therefore, the Session layer does not exist as a distinct layer in the TCP/IP model. While the Physical layer is sometimes grouped into the "Network Access" layer in the 4-layer model, it remains a distinct layer in the widely taught 5-layer TCP/IP model, whereas the Session layer is always excluded as a standalone entity.