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 have seen several examples of encapsulation in this chapter already. The web server encapsulated the home page inside an HTTP header in Figure 2-2. The TCP layer encapsulated the HTTP headers and data inside a TCP header in Figure 2-3. IP encapsulated the TCP headers and the data inside an IP header in Figure 2-4. Finally, the network interface layer encapsulated the IP packets inside both a header and a trailer in Figure 2-5.
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:
