The term encapsulation describes the process of putting headers and trailers around some data. A computer that needs to send data encapsulates the data in headers of the correct format so that the receiving computer will know how to interpret the received data. You can think about the complete process of data encapsulation with TCP/IP as a five-step process. In fact, previous CCNA exams referred to a specific five-step process for encapsulation. This included the typical encapsulation by the application, transport, network, and network interface (referred to as data link) layers as Steps 1 through 4 in the five-step process. The fifth step was the physical layer’s transmission of the bit stream. In case any questions remain in the CCNA question database referring to a five-step encapsulation process, the following list provides the details and explanation. Regardless, the ideas behind the process apply to any networking model and how it encapsulates data:
Step 1 Create the application data and headers—This simply means that the
application has data to send.
Step 2 Package the data for transport—In other words, the transport layer
(TCP or UDP) creates the transport header and places the data behind it.
Step 3 Add the destination and source network layer addresses to the data—
The network layer creates the network header, which includes the
network layer addresses, and places the data behind it.
Step 4 Add the destination and source data link layer addresses to the data—
The data link layer creates the data link header, places the data behind
it, and places the data link trailer at the end.
Step 5 Transmit the bits—The physical layer encodes a signal onto the medium
to transmit the frame.
Step 1 Create the application data and headers—This simply means that the
application has data to send.
Step 2 Package the data for transport—In other words, the transport layer
(TCP or UDP) creates the transport header and places the data behind it.
Step 3 Add the destination and source network layer addresses to the data—
The network layer creates the network header, which includes the
network layer addresses, and places the data behind it.
Step 4 Add the destination and source data link layer addresses to the data—
The data link layer creates the data link header, places the data behind
it, and places the data link trailer at the end.
Step 5 Transmit the bits—The physical layer encodes a signal onto the medium
to transmit the frame.