Senior Acquisitions Editor: Kenyon Brown Development Editor: Kim Wimpsett

Subnetting in Your Head: Class A Addresses

Again, I know this sounds hard, but as with Class C and Class B, the

numbers are the same; we just start in the second octet. What makes this

easy? You only need to worry about the octet that has the largest block

size, which is typically called the interesting octet, and one that is

something other than 0 or 255, such as, for example, 255.255.240.0 (/20)

with a Class A network. The second octet has a block size of 1, so any

number listed in that octet is a subnet. The third octet is a 240 mask,

which means we have a block size of 16 in the third octet. If your host ID

is 10.20.80.30, what is your subnet, broadcast address, and valid host

range?

The subnet in the second octet is 20 with a block size of 1, but the third

octet is in block sizes of 16, so we’ll just count them out: 0, 16, 32, 48, 64,

80, 96… voilà! By the way, you can count by 16s by now, right? Good!

This makes our subnet 10.20.80.0, with a broadcast address of

10.20.95.255 because the next subnet is 10.20.96.0. The valid host range

is 10.20.80.1 through 10.20.95.254. And yes, no lie! You really can do this

in your head if you just get your block sizes nailed!

Let’s practice on one more, just for fun!

Host IP: 10.1.3.65/23

First, you can’t answer this question if you don’t know what a /23 is. It’s

255.255.254.0. The interesting octet here is the third one: 256 – 254 = 2.

Our subnets in the third octet are 0, 2, 4, 6, etc. The host in this question

is in subnet 2.0, and the next subnet is 4.0, so that makes the broadcast

address 3.255. And any address between 10.1.2.1 and 10.1.3.254 is

considered a valid host.

Summary

Did you read Chapters 3 and 4 and understand everything on the first

pass? If so, that is fantastic—congratulations! However, you probably

really did get lost a couple of times. No worries because as I told you,

that’s what usually happens. Don’t waste time feeling bad if you have to

read each chapter more than once, or even 10 times, before you’re truly

good to go. If you do have to read the chapters more than once, you’ll be

seriously better off in the long run even if you were pretty comfortable the

first time through!

This chapter provided you with an important understanding of IP

subnetting—the painless way! And when you’ve got the key material

presented in this chapter really nailed down, you should be able to subnet

IP addresses in your head.

This chapter is extremely essential to your Cisco certification process, so

if you just skimmed it, please go back, read it thoroughly, and don’t forget

to do all the written labs too!

Exam Essentials

Identify the advantages of subnetting. Benefits of subnetting a

physical network include reduced network traffic, optimized network

performance, simplified management, and facilitated spanning of large

geographical distances.

Describe the effect of the

ip subnet-zero

command. This command

allows you to use the first and last subnet in your network design.

Identify the steps to subnet a classful network. Understand how

IP addressing and subnetting work. First, determine your block size by

using the 256-subnet mask math. Then count your subnets and

determine the broadcast address of each subnet—it is always the number

right before the next subnet. Your valid hosts are the numbers between

the subnet address and the broadcast address.

Determine possible block sizes. This is an important part of

understanding IP addressing and subnetting. The valid block sizes are

always 2, 4, 8, 16, 32, 64, 128, etc. You can determine your block size by

using the 256-subnet mask math.

Describe the role of a subnet mask in IP addressing. A subnet

mask is a 32-bit value that allows the recipient of IP packets to

distinguish the network ID portion of the IP address from the host ID

portion of the IP address.

Understand and apply the 2

x

– 2 formula. Use this formula to

determine the proper subnet mask for a particular size network given the

application of that subnet mask to a particular classful network.

Explain the impact of Classless Inter-Domain Routing (CIDR).

CIDR allows the creation of networks of a size other than those allowed

with the classful subnetting by allowing more than the three classful

subnet masks.

Written Labs

In this section, you’ll complete the following labs to make sure you’ve got

the information and concepts contained within them fully dialed in:

Lab 4.1: Written Subnet Practice #1

Lab 4.2: Written Subnet Practice #2

Lab 4.3: Written Subnet Practice #3

You can find the answers to these labs in Appendix A, “Answers to

Written Labs.”

Written Lab 4.1: Written Subnet Practice #1

Write the subnet, broadcast address, and a valid host range for question 1

through question 6. Then answer the remaining questions.

1.  192.168.100.25/30

2.  192.168.100.37/28

3.  192.168.100.66/27

4.  192.168.100.17/29

5.  192.168.100.99/26

6.  192.168.100.99/25

7.  You have a Class B network and need 29 subnets. What is your mask?

8.  What is the broadcast address of 192.168.192.10/29?

9.  How many hosts are available with a Class C /29 mask?

10.  What is the subnet for host ID 10.16.3.65/23?

Written Lab 4.2: Written Subnet Practice #2

Given a Class B network and the net bits identified (CIDR), complete the

following table to identify the subnet mask and the number of host

addresses possible for each mask.

Classful

Address

Subnet

Mask

Number of Hosts per Subnet (2

x

–

2)

/16

/17

/18

/19

/20

/21

/22

/23

/24

/25

/26

/27

/28

/29

/30

Written Lab 4.3: Written Subnet Practice #3

Complete the following based on the decimal IP address.

Decimal IP

Address

Address

Class

Number of

Subnet and

Host Bits

Number

of

Subnets

(2

x

)

Number

of Hosts

(2

x

– 2)

10.25.66.154/23

172.31.254.12/24

192.168.20.123/28

63.24.89.21/18

128.1.1.254/20

208.100.54.209/30

Review Questions

The following questions are designed to test your

understanding of this chapter’s material. For more information on

how to get additional questions, please see

www.lammle.com/ccna

.

You can find the answers to these questions in Appendix B, “Answers to

Review Questions.”

1.  What is the maximum number of IP addresses that can be assigned to

hosts on a local subnet that uses the 255.255.255.224 subnet mask?

A.  14

B.  15

C.  16

D.  30

E.  31

F.  62

2.  You have a network that needs 29 subnets while maximizing the

number of host addresses available on each subnet. How many bits

must you borrow from the host field to provide the correct subnet

mask?

A.  2

B.  3

C.  4

D.  5

E.  6

F.  7

3.  What is the subnetwork address for a host with the IP address

200.10.5.68/28?

A.  200.10.5.56

B.  200.10.5.32

C.  200.10.5.64

D.  200.10.5.0

4.  The network address of 172.16.0.0/19 provides how many subnets and

hosts?

A.  7 subnets, 30 hosts each

B.  7 subnets, 2,046 hosts each

C.  7 subnets, 8,190 hosts each

D.  8 subnets, 30 hosts each

E.  8 subnets, 2,046 hosts each

F.  8 subnets, 8,190 hosts each

5.  Which two statements describe the IP address 10.16.3.65/23? (Choose

two.)

A.  The subnet address is 10.16.3.0 255.255.254.0.

B.  The lowest host address in the subnet is 10.16.2.1 255.255.254.0.

C.  The last valid host address in the subnet is 10.16.2.254

255.255.254.0.

D.  The broadcast address of the subnet is 10.16.3.255 255.255.254.0.

E.  The network is not subnetted.

6.  If a host on a network has the address 172.16.45.14/30, what is the

subnetwork this host belongs to?

A.  172.16.45.0

B.  172.16.45.4

C.  172.16.45.8

D.  172.16.45.12

E.  172.16.45.16

7.  Which mask should you use on point-to-point links in order to reduce

the waste of IP addresses?

A.  /27

B.  /28

C.  /29

D.  /30

E.  /31

8.  What is the subnetwork number of a host with an IP address of

172.16.66.0/21?

A.  172.16.36.0

B.  172.16.48.0

C.  172.16.64.0

D.  172.16.0.0

9.  You have an interface on a router with the IP address of

192.168.192.10/29. Including the router interface, how many hosts

can have IP addresses on the LAN attached to the router interface?

A.  6

B.  8

C.  30

D.  62

E.  126

10.  You need to configure a server that is on the subnet 192.168.19.24/29.

The router has the first available host address. Which of the following

should you assign to the server?

A.  192.168.19.0 255.255.255.0

B.  192.168.19.33 255.255.255.240

C.  192.168.19.26 255.255.255.248

D.  192.168.19.31 255.255.255.248

E.  192.168.19.34 255.255.255.240

11.  You have an interface on a router with the IP address of

192.168.192.10/29. What is the broadcast address the hosts will use

on this LAN?

A.  192.168.192.15

B.  192.168.192.31

C.  192.168.192.63

D.  192.168.192.127

E.  192.168.192.255

12.  You need to subnet a network that has 5 subnets, each with at least 16

hosts. Which classful subnet mask would you use?

A.  255.255.255.192

B.  255.255.255.224

C.  255.255.255.240

D.  255.255.255.248

13.  You configure a router interface with the IP address 192.168.10.62

255.255.255.192 and receive the following error:

Bad mask /26 for address 192.168.10.62

A.  Why did you receive this error?

B.  You typed this mask on a WAN link and that is not allowed.

C.  This is not a valid host and subnet mask combination.

D.

ip subnet-zero

is not enabled on the router.

E.  The router does not support IP.

14.  If an Ethernet port on a router were assigned an IP address of

172.16.112.1/25, what would be the valid subnet address of this

interface?

A.  172.16.112.0

B.  172.16.0.0

C.  172.16.96.0

D.  172.16.255.0

E.  172.16.128.0

15.  Using the following illustration, what would be the IP address of E0 if

you were using the eighth subnet? The network ID is 192.168.10.0/28

and you need to use the last available IP address in the range. The

zero subnet should not be considered valid for this question.

A.  192.168.10.142

B.  192.168.10.66

C.  192.168.100.254

D.  192.168.10.143

E.  192.168.10.126

16.  Using the illustration from the previous question, what would be the

IP address of S0 if you were using the first subnet? The network ID is

192.168.10.0/28 and you need to use the last available IP address in

the range. Again, the zero subnet should not be considered valid for

this question.

A.  192.168.10.24

B.  192.168.10.62

C.  192.168.10.30

D.  192.168.10.127

17.  You have a network in your data center that needs 310 hosts. Which

mask should you use so you waste the least amount of addresses?

A.  255.255.255.0

B.  255.255.254.0

C.  255.255.252.0

D.  255.255.248.0

18.  You have a network with a host address of 172.16.17.0/22. From the

following options, which is another valid host address in the same

subnet?

A.  172.16.17.1 255.255.255.252

B.  172.16.0.1 255.255.240.0

C.  172.16.20.1 255.255.254.0

D.  172.16.16.1 255.255.255.240

E.  172.16.18.255 255.255.252.0

F.  172.16.0.1 255.255.255.0

19.  Your router has the following IP address on Ethernet0: 172.16.2.1/23.

Which of the following can be valid host IDs on the LAN interface

attached to the router? (Choose two.)

A.  172.16.0.5

B.  172.16.1.100

C.  172.16.1.198

D.  172.16.2.255

E.  172.16.3.0

F.  172.16.3.255

20.  Given an IP address 172.16.28.252 with a subnet mask of

255.255.240.0, what is the correct network address?

A.  172.16.16.0

B.  172.16.0.0

C.  172.16.24.0

D.  172.16.28.0

Chapter 5

VLSMs, Summarization, and Troubleshooting

TCP/IP

THE FOLLOWING ICND1 EXAM TOPICS ARE

COVERED IN THIS CHAPTER:

Network Fundamentals

1.7 Apply troubleshooting methodologies to resolve problems

1.7.a Perform fault isolation and document

1.7.b Resolve or escalate

1.7.c Verify and monitor resolution

1.8 Configure, verify, and troubleshoot IPv4 addressing and

subnetting

Now that IP addressing and subnetting have

been thoroughly covered in the last two chapters, you’re fully prepared

and ready to learn all about variable length subnet masks (VLSMs). I’ll

also show you how to design and implement a network using VLSM in

this chapter. After ensuring you’ve mastered VLSM design and

implementation, I’ll demonstrate how to summarize classful boundaries.

We’ll wrap up the chapter by going over IP address troubleshooting,

focusing on the steps Cisco recommends to follow when troubleshooting

an IP network.

So get psyched because this chapter will give you powerful tools to hone

your knowledge of IP addressing and networking and seriously refine the

important skills you’ve gained so far. So stay with me—I guarantee that

your hard work will pay off! Ready? Let’s go!

To find up-to-the minute updates for this chapter, please see

www.lammle.com/ccna

or the book’s web page at

www.sybex.com/go/ccna

.

Variable Length Subnet Masks (VLSMs)

Teaching you a simple way to create many networks from a large single

network using subnet masks of different lengths in various kinds of

network designs is what my primary focus will be in this chapter. Doing

this is called VLSM networking, and it brings up another important

subject I mentioned in Chapter 4, “Easy Subnetting,” classful and

classless networking.

Older routing protocols like Routing Information Protocol version 1

(RIPv1) do not have a field for subnet information, so the subnet

information gets dropped. This means that if a router running RIP has a

subnet mask of a certain value, it assumes that all interfaces within the

classful address space have the same subnet mask. This is called classful

routing, and RIP is considered a classful routing protocol. We’ll cover RIP

and the difference between classful and classless networks later on in

Chapter 9, “IP Routing,” but for now, just remember that if you try to mix

and match subnet mask lengths in a network that’s running an old

routing protocol, such as RIP, it just won’t work!

However, classless routing protocols do support the advertisement of

subnet information, which means you can use VLSM with routing

protocols such as RIPv2, Enhanced Interior Gateway Protocol (EIGRP),

and Open Shortest Path First (OSPF). The benefit of this type of network

is that it saves a bunch of IP address space.

As the name suggests, VLSMs can use subnet masks with different

lengths for different router interfaces. Check out

Figure 5.1

to see an

example of why classful network designs are inefficient.

FIGURE 5.1

Typical classful network

Looking at

Figure 5.1

, you can see that there are two routers, each with

two LANs and connected together with a WAN serial link. In a typical

classful network design that’s running RIP, you could subnet a network

like this:

192.168.10.0 = Network

255.255.255.240 (/28) = Mask

Our subnets would be—you know this part, right?— 0, 16, 32, 48, 64, 80,

etc., which allows us to assign 16 subnets to our internetwork. But how

many hosts would be available on each network? Well, as you know by

now, each subnet provides only 14 hosts, so each LAN has only 14 valid

hosts available (don’t forget that the router interface needs an address too

and is included in the amount of needed valid hosts). This means that one

LAN doesn’t even have enough addresses needed for all the hosts, and

this network as it is shown would not work as addressed in the figure!

Since the point-to-point WAN link also has 14 valid hosts, it would be

great to be able to nick a few valid hosts from that WAN link to give to

our LANs!

All hosts and router interfaces have the same subnet mask—again, known

as classful routing—and if we want this network to be efficient, we would

definitely need to add different masks to each router interface.

But that’s not our only problem—the link between the two routers will

never use more than two valid hosts! This wastes valuable IP address

space, and it’s the big reason you need to learn about VLSM network

design.

VLSM Design

Let’s take

Figure 5.1

and use a classless design instead, which will become

the new network shown in

Figure 5.2

. In the previous example, we wasted

address space—one LAN didn’t have enough addresses because every

router interface and host used the same subnet mask. Not so good. A

better solution would be to provide for only the needed number of hosts

on each router interface, and we’re going to use VLSMs to achieve that

goal.

FIGURE 5.2

Classless network design

Now remember that we can use different size masks on each router

interface. If we use a /30 on our WAN links and a /27, /28, and /29 on

our LANs, we’ll get 2 hosts per WAN interface and 30, 14, and 6 hosts per

LAN interface—nice (remember to count your router interface as a host)!

This makes a huge difference—not only can we get just the right amount

of hosts on each LAN, we still have room to add more WANs and LANs

using this same network!

To implement a VLSM design on your network, you need to

have a routing protocol that sends subnet mask information with the

route updates. The protocols that do that are RIPv2, EIGRP, and

OSPF. Remember, RIPv1 will not work in classless networks, so it’s

considered a classful routing protocol.

Implementing VLSM Networks

To create VLSMs quickly and efficiently, you need to understand how

block sizes and charts work together to create the VLSM masks.

Table 5.1

shows you the block sizes used when creating VLSMs with Class C

networks. For example, if you need 25 hosts, then you’ll need a block size

of 32. If you need 11 hosts, you’ll use a block size of 16. Need 40 hosts?

Then you’ll need a block of 64. You cannot just make up block sizes—

they’ve got to be the block sizes shown in

Table 5.1

. So memorize the

block sizes in this table—it’s easy. They’re the same numbers we used

with subnetting!

Table 5.1

Block sizes

Prefix Mask Hosts Block Size

/25

128

126

128

/26

192

62

64

/27

224

30

32

/28

240

14

16

/29

248

6

8

/30

252

2

4

The next step is to create a VLSM table.

Figure 5.3

shows you the table

used in creating a VLSM network. The reason we use this table is so we

don’t accidentally overlap networks.

You’ll find the sheet shown in

Figure 5.3

very valuable because it lists

every block size you can use for a network address. Notice that the block

sizes start at 4 and advance all the way up to a block size of 128. If you

have two networks with block sizes of 128, you can have only 2 networks.

With a block size of 64, you can have only 4, and so on, all the way to 64

networks using a block size of 4. Of course, this is assuming you’re using

the

ip subnet-zero

command in your network design.

So now all you need to do is fill in the chart in the lower-left corner, then

add the subnets to the worksheet and you’re good to go!

Based on what you’ve learned so far about block sizes and the VLSM

table, let’s create a VLSM network using a Class C network address

192.168.10.0 for the network in

Figure 5.4

, then fill out the VLSM table,

as shown in

Figure 5.5

.

In

Figure 5.4

, we have four WAN links and four LANs connected

together, so we need to create a VLSM network that will save address

space. Looks like we have two block sizes of 32, a block size of 16, and a

block size of 8, and our WANs each have a block size of 4. Take a look and

see how I filled out our VLSM chart in

Figure 5.5

.