Objectives–Cisco Network Design and the Access Layer
Upon completion of this chapter, you will be able to answer the following questions:
- What is the process of encapsulation and Ethernet framing?
- What is the function at each layer of the three-layer network design model?
- How do you improve network communication at the access layer?
- Why is it important to contain broadcasts within a network?
Key Terms
This chapter uses the following key terms. You can find the definitions in the Glossary.
Address Resolution Protocol (ARP) page 129
protocol data units (PDUs) page 114
Introduction (6.0.1)
At this point, you know about protocols and why they are important for network communication. To keep the analogy of sending a card to your relative going, think about the card, and the envelope, and your relative‛s address, as well as your own address. All of these parts are required to correctly deliver the card from your house to your relative‛s house. This chapter helps you to understand the different types of network addresses and the different parts of a network address. You will use this knowledge every day of your career, so don‛t waste any time.
Encapsulation and the Ethernet Frame (6.1)
Ethernet is a protocol used to deliver information from one Ethernet network interface card (NIC) to another Ethernet NIC on the same network. This section explains the process known as encapsulation and how the fields of an Ethernet frame are used to transmit the embedded information.
Video—The Fields of the Ethernet Frame (6.1.1)
Refer to the online course to view this video.
Encapsulation (6.1.2)
When sending a letter, a letter writer uses an accepted format to ensure that the letter is delivered and understood by the recipient. In the same way, a message that is sent over a computer network follows specific format rules for it to be delivered and processed.
The process of placing one message format (the letter) inside another message format (the envelope) is called encapsulation. De-encapsulation occurs when the process is reversed by the recipient and the letter is removed from the envelope. Just as a letter is encapsulated in an envelope for delivery, computer messages are encapsulated.
Each computer message is encapsulated in a specific format, called a frame, before it is sent over the network. A frame acts like an envelope; it provides the address of the intended destination and the address of the source host. The format and contents of a frame are determined by the type of message being sent and the channel over which it is communicated. Messages that are not correctly formatted are not successfully delivered to or processed by the destination host.
A common example of requiring the correct format in human communication is when sending a letter, as shown in Figure 6-1. An envelope has the address of the sender and receiver, each located at the proper place. If the destination address and formatting are not correct, the letter is not delivered.
Figure 6-1 Format for Sending a Letter
Similar to sending a letter, a message that is sent over a computer network follows specific format rules for it to be delivered and processed.
Internet Protocol (IP) is a protocol with a similar function to the envelope example. In Figure 6-2, the fields of the Internet Protocol version 6 (IPv6) packet identify the source of the packet and its destination. IP is responsible for sending a message from the message source to a destination over one or more networks.
Figure 6-2 Fields in an IPv6 Header
The fields of the IPv6 packet are discussed in detail in Chapter 10, “IPv4 and IPv6 Address Management.”