The process of placing one PDU inside of another PDU is called encapsulation.

When data moves down the networking stack (for example, from the application layer to the transport, network, and then data link layers), each layer encapsulates the PDU from the layer above by adding its own header (and sometimes a trailer). For instance, an application message is encapsulated into a transport segment, which is encapsulated into a network packet, which is then encapsulated into a data link frame before being sent over the medium.

In networking exam questions, if you see “What process involves placing one PDU inside of another PDU?”, the correct multiple‑choice answer is encapsulation , not segmentation, encoding, or flow control.

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