In the last section, I have explaint that data will need to pass through four layers in TCP/IP models before it will be sent out of the computer. As data from application layer is passed down the protocol stack, different types are information are added. This process is commonly as encapsulation. These information are important during the delivery of data to the destination.It is the same as the sweet packaging example. The sweets are put inside the bags, the bags are packed into the boxes and the boxes are put inside the cars. The bags, boxes and cars normally will contain information that are needed for the delivery.The sweet packaging example uses words like "Bag", "Box" and "Car" at each stage of packaging. For network, the form that a piece of data takes at any layer is called a Protocol Data Unit (PDU):
Data - The general term for the PDU used at the Application layer
Segment - Transport Layer PDU
Packet - Internetwork Layer PDU
Frame - Network Access Layer PDU
Bits - A PDU used when physically transmitting data over the medium
Once the data reaches the destination, the process of decapsulation will start. The data is decapsulated as it moves up the stack toward application layer. Application will only need the data, so information that are needed for the delivery of the data are removed.