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CompTIA Network+ Rapid Review: Network Concepts

This chapter from CompTIA Network+ Rapid Review (Exam N10-005) covers topics to help you prepare for Exam N10-005 such as the OSI reference model, basic networking functions such as IP addressing, and more.

The Network Concepts domain accounts for approximately 21% of the CompTIA Network+ exam, but more than that, it covers some of the most fundamental principles of computer networking. These are concepts that you encounter repeatedly, both as you prepare for the exam and as you work in the IT field.

To excel at this objective, you must possess a good grasp of certain organizational concepts, such as the OSI reference model; an understanding of basic networking functions, such as IP addressing; and some memorized facts and figures, such as well-known port numbers.

This chapter covers the following objectives:

  • Objective 1.1: Compare the layers of the OSI and TCP/IP models
  • Objective 1.2: Classify how applications, devices, and protocols relate to the OSI model layers
  • Objective 1.3: Explain the purpose and properties of IP addressing
  • Objective 1.4: Explain the purpose and properties of routing and switching
  • Objective 1.5: Identify common TCP and UDP default ports
  • Objective 1.6: Explain the function of common networking protocols
  • Objective 1.7: Summarize DNS concepts and components
  • Objective 1.8: Given a scenario, implement proper network troubleshooting methodology
  • Objective 1.9: Identify virtual network components

Objective 1.1: Compare the layers of the OSI and TCP/IP models

For this exam objective, you must know the structure of the two basic models defining the networking process: the OSI reference model and the TCP/IP model. The OSI model is designed to be independent of any specific networking implementation, and as a result, it does not conform precisely to the networking stacks in general use today. By contrast, the TCP/IP model was designed with specific protocols in mind, and is pertinent only on networks using those protocols. However, virtually all networks today use TCP/IP, so the TCP/IP model is as viable as the OSI model for demonstration, documentation, and discussion purposes.

Exam need to know

  • OSI model

    For example: Do you know that the OSI reference model consists of seven layers: Layer 1 – Physical; Layer 2 – Data link; Layer 3 – Network; Layer 4 – Transport; Layer 5 – Session; Layer 6 – Presentation; and Layer 7 – Application?

  • TCP/IP model

    For example: Do you know that the model of the TCP/IP protocol stack consists of four layers: the network interface layer (also known as the link layer); the internet layer; the transport layer; and the application layer?

OSI model

The Open Systems Interconnection (OSI) reference model is a theoretical example of a network protocol stack, which networking educators and administrators use to categorize and define a computer’s various networking functions. The top of the model interacts with the applications running on the computer, which might at times require the services of the network. The bottom of the model connects to the network medium over which the system transmits its signals, as shown in Figure 1-1. There are different protocols operating at the various layers of the model, each of which provides functions needed to complete the network communication process.

Figure 1-1

Figure 1-1 The seven layers of the OSI reference model.

True or false: The layers of the OSI reference model correspond to the initials of the mnemonic “All People Seem To Need Data Processing.”

Answer: True. The layers of the OSI model, from top to bottom, are application, presentation, session, transport, network, data-link, and physical.

TCP/IP model

The development of the TCP/IP protocols began years before the documents defining the OSI reference model were published, but the protocols conform to a layered model in much the same way. Instead of the seven layers used by the OSI model, the TCP/IP model—sometimes called the Department of Defense (DoD) model—has four layers. The TCP/IP model layers, in comparison with those of the OSI model, are shown in Figure 1-2.

Figure 1-2

Figure 1-2 The four TCP/IP model layers, compared with the seven-layer OSI reference model.

The TCP/IP model layers—even those with the same names—are not exactly analogous to the OSI model layers, nor were the models created with the same intent. The OSI model is intended to be a guide for the creation of networking protocols, whereas the TCP/IP model is a representation of protocols that already exist.

True or false: The link layer of the TCP/IP protocol stack is exactly congruent to the data-link and physical layers of the OSI model.

Answer: False. Despite being roughly analogous to the OSI data-link layer, the TCP/IP link layer does not include physical specifications of any kind, nor does it include complex LAN protocols such as Ethernet. Therefore, on many TCP/IP networks, the protocol operating at the link layer might not be part of the TCP/IP suite.

True or false: The TCP/IP protocol stack was designed to conform to the OSI reference model.

Answer: False. Most of the TCP/IP protocols that make up the protocol stack were designed and developed in the 1970s, and therefore predate the OSI reference model. In fact, there is no protocol stack in common use that conforms precisely to the OSI layers. Although originally intended to be a model for an actual networking solution, OSI is now used only as an educational and organizational tool.

Can you answer these questions?

Find the answers to these questions at the end of this chapter.

  1. What is the primary TCP/IP protocol operating at the link layer of the TCP/IP model?
  2. Which of the OSI model layers do not have TCP/IP protocols directly associated with them?
  3. What are the two protocols operating at the transport layer in both the OSI and TCP/IP models?
  4. What organizations were responsible for publishing the original documents defining the OSI reference model and the TCP/IP model?

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