CCENT-CCNA：ICND1 100-105 Certification Guide
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TCP/IP protocol model

When considering the fact that communication between two computers consists of certain steps and that these steps in the world of computer networks are known as layers of communications, it will not be that hard to understand the layered nature of the TCP/IP protocol model. Named according to two main protocols like IP and TCP, the TCP/IP protocol model consists of four layers known as network access, internet, transport, and application. In addition to IP and TCP protocols, there are other protocols that comprise the TCP/IP protocol suite. So, to not confuse the TCP/IP protocol suite with the TCP/IP protocol model, consider the fact that the TCP/IP protocol suite is a set of protocols where IP and TCP protocols are dominant. Also, the TCP/IP protocol suite is known as the language of the internet.

Figure 2.10 represents the communication between PC1 and PC2, as observed by the TCP/IP protocol model. The following takes place in each communication layer in top-down order:

  • When the user in PC1 uses a browser to access a certain website on the internet, this enables an interaction between a proper protocol in the application layer and the browser. Once the request of a browser to communicate data over the network has been processed by an HTTP protocol, the request is then forwarded to the next lower layer.
  • In the transport layer, TCP protocol is taking care to package the data coming from the HTTP protocol into segments called datagrams before then forwarding them to the lower layer.
  • In the internet layer, on the received datagrams, the IP protocol adds the source and destination IP addresses, and thus the data is represented by packets. Thereafter, packets are transmitted to the lower layer of the TCP/IP protocol model, which is known as network access.
  • In the network access layer, on the received packets, the Ethernet protocols (see section, Ethernet communication technology, later in this chapter) are organizing the data into frames by adding the source and destination MAC addresses. Then, these frames are carried over to the network so that they can reach the destination:
Figure 2.10. TCP/IP protocol model
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