Chapter 17. What Is Multicasting?

This chapter covers the following topics:

  • IP Styles of Addressing—Explains the different delivery mechanisms that may be specified by an IP address (unicast, broadcast, or multicast).

  • Multicast Addressing—Describes a multicast IP address and the process used to convert a multicast IP to a multicast MAC address.

  • Current Multicast Usage—Lists some of the ways that multicasts are used and some of the problems that limit the use of multicasting.

Multicast traffic is typically sent by one source and received by a group of recipients that might be spread throughout a network and that might change over time. Examples of multicast traffic include video streams for instruction or entertainment, certain audio conference calls, and one-to-many PC file-imaging applications.

This chapter outlines the differences between unicast, broadcast, and multicast packet types. It then describes multicast addressing, including multicast IP and multicast MAC addresses. You then learn about how today's applications use multicasting and the related challenges that you need to be aware of.

The purpose of the "Do I Know This Already?" quiz is to help you decide which parts of this chapter to use. If you already intend to read the entire chapter, you do not necessarily need to answer these questions.

The 10-question quiz, derived from the major sections in the "Foundation Topics" portion of the chapter, helps you determine how to spend your limited study time.

Table 17-1 outlines the major topics discussed in this chapter and the corresponding quiz questions.

Table 17-1. "Do I Know This Already?" Foundation Topics Section-to-Question Mapping
Foundation Topics SectionQuestions Covered in This SectionScore
IP Styles of Addressing1–3 
Multicast Addressing4–8 
Current Multicast Usage9–10 
Total Score  


The goal of self-assessment is to gauge your mastery of the topics in this chapter. If you do not know the answer to a question or are only partially sure of the answer, you should mark the question wrong for purposes of the self-assessment. Giving yourself credit for an answer you correctly guessed skews your results and might provide you with a false sense of security.

1.Which of the following describes a difference between broadcasts and multicasts?
  1. Multicasts are one-to-many.

  2. Multicasts are unidirectional.

  3. Multicasts are routable.

  4. Multicasts are used by RIPv1.

2.Multicasting is efficient in terms of processor resources for which of the following reasons?
  1. Multicasts can be ignored by the network card.

  2. Multicasts have a specific set of IP addresses.

  3. Multicasts are not routable.

  4. Multicasts do not support TCP.

3.How do Layer 2 switches treat multicast traffic by default?
  1. Discard it

  2. ARP for the MAC

  3. Ignore it

  4. Flood it

4.Which one of the following is a multicast address?



  4. corresponds to which of the following MAC addresses?
  1. 0102.0300.0000

  2. 0100.5e01.0203

  3. e000.0001.0203

  4. 1000.5e01.0203

6.How many unique multicast IP addresses can correspond to one multicast MAC address?
  1. 1

  2. 2

  3. 8

  4. 32

7.Which one of the following multicast addresses does not share a multicast MAC address?




8.Which of the following is the OUI for multicast MAC addresses?
  1. 015E00

  2. 01005E

  3. 00015E

  4. 5E0100

9.Which of the following is not a capability of UDP multicast?
  1. Packet ordering

  2. Retransmission

  3. Limiting receivers

  4. Dynamic membership

10.Multicasting supports applications that communicate
  1. one-to-one

  2. one-to-many

  3. many-to-many

  4. many-to-one

You can find the answers to the "Do I Know This Already?" quiz in Appendix A, "Answers to Chapter 'Do I Know This Already?' Quizzes and Q&A Sections." The suggested choices for your next step are as follows:

  • 7 or less overall score—Read the entire chapter. This includes the "Foundation Topics," "Foundation Summary," and "Q&A" sections.

  • 8 or 9—Begin with the "Foundation Summary" section and then go to the "Q&A" section at the end of the chapter. If you have trouble with these exercises, read the appropriate sections in "Foundation Topics."

  • 10—If you want more review on these topics, skip to the "Foundation Summary" section and then go to the "Q&A" section at the end of the chapter. Otherwise, move to the next chapter.