Ccna routing and Switching Complete Study Guide

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Chapter 3 

■

  Introduction to TCP/IP 

Multicast Address

Multicast is a different beast entirely. At first glance, it appears to be a hybrid of uni-

cast and broadcast communication, but that isn’t quite the case. Multicast does allow 

point-to-multipoint communication, which is similar to broadcasts, but it happens in a 

different manner. The crux of multicast is that it enables multiple recipients to receive 

messages without flooding the messages to all hosts on a broadcast domain. However, 

this is not the default behavior—it’s what we can do with multicasting if it’s configured 

correctly!

Multicast works by sending messages or data to IP multicast group addresses. Unlike 

with broadcasts, which aren’t forwarded, routers then forward copies of the packet out to 

every interface that has hosts subscribed to that group address. This is where multicast dif-

fers from broadcast messages—with multicast communication, copies of packets, in theory, 

are sent only to subscribed hosts. For example, when I say in theory, I mean that the hosts 

will receive a multicast packet destined for 224.0.0.10. This is an EIGRP packet, and only 

a router running the EIGRP protocol will read these. All hosts on the broadcast LAN, and 

Ethernet is a broadcast multi-access LAN technology, will pick up the frame, read the des-

tination address, then immediately discard the frame unless they’re in the multicast group. 

This saves PC processing, not LAN bandwidth. Be warned though—multicasting can cause 

some serious LAN congestion if it’s not implemented carefully! Figure 3.24 shows a Cisco 

router sending an EIGRP multicast packet on the local LAN and only the other Cisco 

router will accept and read this packet.

f I g u r e   3 . 2 4     EIGRP multicast example

We see that

multicast frame, but

this is not for us!

I run EIGRP! This is

for me!

I have EIGRP data!

224.0.0.10

There are several different groups that users or applications can subscribe to. The range 

of multicast addresses starts with 224.0.0.0 and goes through 239.255.255.255. As you 

can see, this range of addresses falls within IP Class D address space based on classful IP 

assignment.

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Exam Essentials 

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Summary

If you made it this far and understood everything the first time through, you should be 

extremely proud of yourself! We really covered a lot of ground in this chapter, but understand 

that the information in it is critical to being able to navigate well through the rest of this book.

If you didn’t get a complete understanding the first time around, don’t stress. It really 

wouldn’t hurt you to read this chapter more than once. There is still a lot of ground to 

cover, so make sure you’ve got this material all nailed down. That way, you’ll be ready for 

more, and just so you know, there’s a lot more! What we’re doing up to this point is build-

ing a solid foundation to build upon as you advance.

With that in mind, after you learned about the DoD model, the layers, and associated pro-

tocols, you learned about the oh-so-important topic of IP addressing. I discussed in detail the 

difference between each address class, how to find a network address and broadcast address, 

and what denotes a valid host address range. I can’t stress enough how important it is for you 

to have this critical information unshakably understood before moving on to Chapter 4!

Since you’ve already come this far, there’s no reason to stop now and waste all those 

brainwaves and new neural connections. So don’t stop—go through the written labs and 

review questions at the end of this chapter and make sure you understand each answer’s 

explanation. The best is yet to come!

Exam Essentials