At which OSI layer is a source port number added to a PDU during the encapsulation process?

The source port number is added to a Protocol Data Unit (PDU) during the encapsulation process at the Transport layer of the OSI model. The Transport layer is responsible for providing reliable communication between two hosts, which includes managing the flow of data and ensuring that it is transported correctly.

At this layer, segmenting the data into smaller pieces occurs, and each segment is assigned a source port number and a destination port number. These port numbers are crucial for distinguishing between different applications or services that are using the transport layer on a host. This allows multiple applications to communicate over the network at the same time without interference.

The source port number identifies the application originating the data, while the destination port number identifies the intended application on the receiving host. By including these port numbers in the Transport layer PDU, the network can ensure that the data is delivered to the correct application, thus allowing for effective communication between applications on different computers.