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Foundation Summary 135

Foundation Summary

The “Foundation Summary” section lists the most important facts from the chapter. Although this section does not list everything that will be on the exam, a well-prepared CCNA candidate should at a minimum know all the details in each Foundation Summary before taking the exam.

The thought process used to answer questions about the number of hosts and subnets in a network, based on a network number and a subnet mask, is summarized in the following list:

Step 1 Identify the structure of the IP address.

Step 2 Identify the size of the network part of the address based on Class A, B, and C rules.

Step 3 Identify the size of the host part of the address based on the number of binary 0s in the mask.

Step 4 The size of the subnet part is what’s “left over”; mathematically, it is 32

– (number of network + host bits).

Step 5 Declare the number of subnets, which is 2number-of-subnet-bits – 2.

Step 6 Declare the number of hosts per subnet, which is 2number-of-host-bits – 2.

Here’s a formal definition of the “algorithm” to find the first and last IP addresses in a subnet when you know the subnet number and broadcast addresses:

For the first valid IP address, copy the subnet number, but add 1 to the fourth octet.

For the last valid IP address, copy the broadcast address, but subtract 1 from the fourth octet.

The range of valid IP addresses starts with the first number and ends with the last.

To find the subnet number, perform a Boolean AND between the address and the subnet mask, as shown in Table 4-36.

Table 4-36 Bitwise Boolean AND Example

 

Decimal

Binary

 

 

 

Address

150.150.2.1

1001 0110 1001 0110 0000 0010 0000 0001

 

 

 

Mask

255.255.255.0

1111 1111 1111 1111 1111 1111 0000 0000

 

 

 

Result of AND

150.150.2.0

1001 0110 1001 0110 0000 0010 0000 0000

 

 

 

136 Chapter 4: IP Addressing and Subnetting

To find the subnet broadcast address, take the subnet number in binary and change all the host bits to binary 1s.

The following three-step process lists all the subnet numbers of a network. This process refers to Table 4-37.

Step 1 Write down the network number and subnet mask in the first two rows of the subnet list chart.

Step 2 Write down the network number in the third row. This is the zero subnet, which is one of the two reserved subnets.

Step 3 Do the following two tasks, stopping when the next number you would write down in the interesting column is 256. (But don’t write it down— it’s invalid.)

(a)Copy all three uninteresting octets from the previous line.

(b)Add the magic number to the previous interesting octet, and write it down as the value of the interesting octet.

Table 4-37 Subnet List Chart: 130.4.0.0/255.255.252.0

Octet

1

2

3

4

Network Number

Mask

Subnet Zero

First Subnet

Next Subnet

(Skipping Many Subnets)

Last Subnet

Broadcast Subnet

Q&A 137

Q&A

As mentioned in the Introduction, you have two choices for review questions. The following questions give you a bigger challenge than the exam because they are open-ended. By reviewing with this more-difficult question format, you can exercise your memory better and prove your conceptual and factual knowledge of the topics covered in this chapter. The answers to these questions are found in appendix A.

If you want more questions and practice with subnetting, you have a couple of options. You can look at the chapter on the CD titled “Subnetting Practice: 25 Subnetting Questions.”

Also, if you bought the two-book set, and you already own the CCNA INTRO Exam Certification Guide, you can look at the questions at the end of Chapter 4, “Fundamentals of WANs,” of that book. Chapter 4 of that book repeats what is discussed in this chapter. However, more than half of the questions at the end of that chapter are different from the ones in this chapter, so you can get some more practice.

For more practice with exam-like question formats, including multiple-choice questions and those using a router simulator, use the exam engine on the CD.

1.Name the parts of an IP address.

2.Define subnet mask. What do the bits in the mask whose values are binary 0 tell you about the corresponding IP address(es)?

3.Given the IP address 10.5.118.3 and the mask 255.255.0.0, what is the subnet number?

4.Given the IP address 190.1.42.3 and the mask 255.255.255.0, what is the subnet number?

5.Given the IP address 140.1.1.1 and the mask 255.255.255.248, what is the subnet number?

6.Given the IP address 167.88.99.66 and the mask 255.255.255.192, what is the subnet number?

7.Given the IP address 10.5.118.3 and the mask 255.255.0.0, what is the broadcast address?

8.Given the IP address 190.1.42.3 and the mask 255.255.255.0, what is the broadcast address?

9.Given the IP address 140.1.1.1 and the mask 255.255.255.248, what is the broadcast address?

138Chapter 4: IP Addressing and Subnetting

10.Given the IP address 167.88.99.66 and the mask 255.255.255.192, what is the broadcast address?

11.Given the IP address 10.5.118.3 and the mask 255.255.0.0, what are the assignable IP addresses in this subnet?

12.Given the IP address 190.1.42.3 and the mask 255.255.255.0, what are the assignable IP addresses in this subnet?

13.Given the IP address 140.1.1.1 and the mask 255.255.255.248, what are the assignable IP addresses in this subnet?

14.Given the IP address 167.88.99.66 and the mask 255.255.255.192, what are the assignable IP addresses in this subnet?

15.Given the IP address 10.5.118.3 and the mask 255.255.255.0, what are all the subnet numbers if the same (static) mask is used for all subnets in this network?

16.How many IP addresses can be assigned in each subnet of 10.0.0.0, assuming that a mask of 255.255.255.0 is used? If the same (static) mask is used for all subnets, how many subnets are there?

17.How many IP addresses can be assigned in each subnet of 140.1.0.0, assuming that a mask of 255.255.255.248 is used? If the same (static) mask is used for all subnets, how many subnets are there?

18.You design a network for a customer who wants the same subnet mask on every subnet. The customer will use network 10.0.0.0 and needs 200 subnets, each with 200 hosts maximum. What subnet mask would you use to allow the most growth in subnets? Which mask would work and would allow for the most growth in the number of hosts per subnet?

19.Refer to Figure 4-5. Fred is configured with IP address 10.1.1.1. Router A’s Ethernet interface is configured with 10.1.1.100. Router A’s serial interface uses 10.1.1.101. Router B’s serial interface uses 10.1.1.102. Router B’s Ethernet uses 10.1.1.200. The web server uses 10.1.1.201. Mask 255.255.255.192 is used in all cases. Is anything wrong with this network? What is the easiest thing you could do to fix it? You may assume any working interior routing protocol.

Q&A 139

Figure 4-5 Sample Network for Subnetting Questions

A B

Web

Fred

20.Refer to Figure 4-5. Fred is configured with IP address 10.1.1.1, mask 255.255.255.0. Router A’s Ethernet is configured with 10.1.1.100, mask 255.255.255.224. Router A’s serial interface uses 10.1.1.129, mask 255.255.255.252. Router B’s serial interface uses

10.1.1.130, mask 255.255.255.252. Router B’s Ethernet uses 10.1.1.200, mask 255.255.255.224. The web server uses 10.1.1.201, mask 255.255.255.224. Is anything wrong with this network? What is the easiest thing you could do to fix it? You may assume any working interior routing protocol.

21.Refer to Figure 4-5. Fred is configured with IP address 10.1.1.1, mask 255.255.255.0. Router A’s Ethernet is configured with 10.1.1.100, mask 255.255.255.224. Router A’s serial interface uses 10.1.1.129, mask 255.255.255.252. Router B’s serial interface uses 10.1.1.130, mask 255.255.255.252. Router B’s Ethernet uses 10.1.1.200, mask 255.255.255.224. The web server uses 10.1.1.201, mask 255.255.255.224. Is anything wrong with this network? What is the easiest thing you could do to fix it? You may assume any working interior routing protocol.

22.Refer to Figure 4-5. Fred is configured with IP address 10.1.1.1, mask 255.255.255.240. Router A’s Ethernet is configured with 10.1.1.2, mask 255.255.255.240. Router A’s serial interface uses 10.1.1.129, mask 255.255.255.252. Router B’s serial interface uses 10.1.1.130, mask 255.255.255.252. Router B’s Ethernet uses 10.1.1.200, mask 255.255.255.128. The web server uses 10.1.1.201, mask 255.255.255.128. Is anything wrong with this network? What is the easiest thing you could do to fix it? You may assume any working interior routing protocol.

This chapter covers the following subjects:

Configuring and Testing Static Routes

Distance Vector Concepts

Configuring RIP and IGRP

C H A P T E R 5

RIP, IGRP, and Static Route

Concepts and Configuration

Routers forward IP packets based on the destination IP address in the IP packet header. They compare the destination address to the routing table with the hope of finding a matching entry—an entry that tells the router where to forward the packet next. If the router does not match an entry in the routing table, and no default route exists, the router discards the packet. Therefore, having a full and accurate routing table is important.

Chapters 5, 6, “OSPF and EIGRP Concepts and Configuration,” and 7, “Advanced Routing Protocol Topics,” cover the concepts and configuration required to fill a router’s routing table. Cisco expects CCNAs to demonstrate a comfortable understanding of the logic behind the routing of packets and the different but related logic behind routing protocols—the protocols used to discover routes. To fully appreciate the nuances of routing protocols, you need a thorough understanding of routing—the process of forwarding packets. If you bought both my CCNA books, refer to Chapter 5, “Fundamentals of IP,” of CCNA INTRO Exam Certification Guide for a brief review.

AUTHOR’S NOTE If you happen to not be reading the INTRO Exam Certification Guide as well, you may want to refer to appendix D, Comparisons of Dynamic Routing Protocols, for a brief overview of a few other routing protocols. For those of you who are preparing for the CCNA exam and are using the INTRO Exam Certification Guide as well, those small details are covered in the other book.

Before diving into routing protocol details, this chapter starts with a short section about how to configure static routes. Thinking through static route configuration can help people new to routing better understand routing and routing protocols. Following that, the text covers the most important topics in this chapter—distance vector concepts, Routing Information Protocol (RIP), and Interior Gateway Routing Protocol (IGRP).

Chapter 6 covers the most basic concepts and configuration of Open Shortest Path First (OSPF) and Enhanced IGRP (EIGRP). Chapter 7 covers several advanced topics related to routing protocols.