Which of the following protocols is not part of OSI Layer 4?

The rationale for identifying the protocol that does not belong to OSI Layer 4 centers around understanding the functions and roles of the various layers in the OSI model.

Layer 4 of the OSI model is designated for the Transport layer, which is responsible for end-to-end communication and ensures reliable data transfer between hosts. The protocols within this layer include Transmission Control Protocol (TCP), User Datagram Protocol (UDP), and Stream Control Transmission Protocol (SCTP). These protocols are designed for managing the transmission of data segments between applications, establishing connections, and providing error-checking mechanisms.

In contrast, the Internet Protocol (IP) operates at Layer 3, known as the Network layer. The primary function of IP is to handle addressing and routing of packets across network boundaries, ensuring that data is forwarded to the correct destination based on its IP address. Since IP is responsible for routing packets and does not engage in the transport layer responsibilities, it is correctly classified as not belonging to Layer 4 of the OSI model.

This differentiation clarifies the hierarchical structure of the OSI model, where each protocol has a specific role tailored to the requirements and functionalities of the layer it operates within.