Senior Acquisitions Editor: Kenyon Brown Development Editor: Kim Wimpsett

Compare and contrast UDP and TCP characteristics and

features. TCP is connection-oriented, acknowledged, and sequenced

and has flow and error control, while UDP is connectionless,

unacknowledged, and not sequenced and provides no error or flow

control.

Understand the role of port numbers. Port numbers are used to

identify the protocol or service that is to be used in the transmission.

Identify the role of ICMP. Internet Control Message Protocol (ICMP)

works at the Network layer and is used by IP for many different services.

ICMP is a management protocol and messaging service provider for IP.

Define the Class A IP address range. The IP range for a Class A

network is 1–126. This provides 8 bits of network addressing and 24 bits

of host addressing by default.

Define the Class B IP address range. The IP range for a Class B

network is 128–191. Class B addressing provides 16 bits of network

addressing and 16 bits of host addressing by default.

Define the Class C IP address range. The IP range for a Class C

network is 192 through 223. Class C addressing provides 24 bits of

network addressing and 8 bits of host addressing by default.

Identify the private IP ranges. The Class A private address range is

10.0.0.0 through 10.255.255.255. The Class B private address range is

172.16.0.0 through 172.31.255.255. The Class C private address range is

192.168.0.0 through 192.168.255.255.

Understand the difference between a broadcast, unicast, and

multicast address. A broadcast is to all devices in a subnet, a unicast is

to one device, and a multicast is to some but not all devices.

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 3.1: TCP/IP

Lab 3.2: Mapping Applications to the DoD Model

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

Written Labs.”

Written Lab 3.1: TCP/IP

Answer the following questions about TCP/IP:

1.  What is the Class C address range in decimal and in binary?

2.  What layer of the DoD model is equivalent to the Transport layer of

the OSI model?

3.  What is the valid range of a Class A network address?

4.  What is the 127.0.0.1 address used for?

5.  How do you find the network address from a listed IP address?

6.  How do you find the broadcast address from a listed IP address?

7.  What is the Class A private IP address space?

8.  What is the Class B private IP address space?

9.  What is the Class C private IP address space?

10.  What are all the available characters that you can use in hexadecimal

addressing?

Written Lab 3.2: Mapping Applications to the DoD Model

The four layers of the DoD model are Process/Application, Host-to-Host,

Internet, and Network Access. Identify the layer of the DoD model on

which each of these protocols operates.

1.  Internet Protocol (IP)

2.  Telnet

3.  FTP

4.  SNMP

5.  DNS

6.  Address Resolution Protocol (ARP)

7.  DHCP/BootP

8.  Transmission Control Protocol (TCP)

9.  X Window

10.  User Datagram Protocol (UDP)

11.  NFS

12.  Internet Control Message Protocol (ICMP)

13.  Reverse Address Resolution Protocol (RARP)

14.  Proxy ARP

15.  TFTP

16.  SMTP

17.  LPD

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 must happen if a DHCP IP conflict occurs?

A.  Proxy ARP will fix the issue.

B.  The client uses a gratuitous ARP to fix the issue.

C.  The administrator must fix the conflict by hand at the DHCP

server.

D.  The DHCP server will reassign new IP addresses to both

computers.

2.  Which of the following Application layer protocols sets up a secure

session that’s similar to Telnet?

A.  FTP

B.  SSH

C.  DNS

D.  DHCP

3.  Which of the following mechanisms is used by the client to avoid a

duplicate IP address during the DHCP process?

A.  Ping

B.  Traceroute

C.  Gratuitous ARP

D.  Pathping

4.  What protocol is used to find the hardware address of a local device?

A.  RARP

B.  ARP

C.  IP

D.  ICMP

E.  BootP

5.  Which of the following are layers in the TCP/IP model? (Choose

three.)

A.  Application

B.  Session

C.  Transport

D.  Internet

E.  Data Link

F.  Physical

6.  Which class of IP address provides a maximum of only 254 host

addresses per network ID?

A.  Class A

B.  Class B

C.  Class C

D.  Class D

E.  Class E

7.  Which of the following describe the DHCP Discover message? (Choose

two.)

A.  It uses ff:ff:ff:ff:ff:ff as a layer 2 broadcast.

B.  It uses UDP as the Transport layer protocol.

C.  It uses TCP as the Transport layer protocol.

D.  It does not use a layer 2 destination address.

8.  Which layer 4 protocol is used for a Telnet connection?

A.  IP

B.  TCP

C.  TCP/IP

D.  UDP

E.  ICMP

9.  Private IP addressing was specified in RFC __________ .

10.  Which of the following services use TCP? (Choose three.)

A.  DHCP

B.  SMTP

C.  SNMP

D.  FTP

E.  HTTP

F.  TFTP

11.   Which Class of IP addresses uses the pattern shown here?

A.  Class A

B.  Class B

C.  Class C

D.  Class D

12.  Which of the following is an example of a multicast address?

A.  10.6.9.1

B.  192.168.10.6

C.  224.0.0.10

D.  172.16.9.5

13.  The following illustration shows a data structure header. What

protocol is this header from?

A.  IP

B.  ICMP

C.  TCP

D.  UDP

E.  ARP

F.  RARP

14.  If you use either Telnet or FTP, what layer are you using to generate

the data?

A.  Application

B.  Presentation

C.  Session

D.  Transport

15.  The DoD model (also called the TCP/IP stack) has four layers. Which

layer of the DoD model is equivalent to the Network layer of the OSI

model?

A.  Application

B.  Host-to-Host

C.  Internet

D.  Network Access

16.  Which two of the following are private IP addresses?

A.  12.0.0.1

B.  168.172.19.39

C.  172.20.14.36

D.  172.33.194.30

E.  192.168.24.43

17.  What layer in the TCP/IP stack is equivalent to the Transport layer of

the OSI model?

A.  Application

B.  Host-to-Host

C.  Internet

D.  Network Access

18.  Which statements are true regarding ICMP packets? (Choose two.)

A.  ICMP guarantees datagram delivery.

B.  ICMP can provide hosts with information about network

problems.

C.  ICMP is encapsulated within IP datagrams.

D.  ICMP is encapsulated within UDP datagrams.

19.  What is the address range of a Class B network address in binary?

A.  01xxxxxx

B.  0xxxxxxx

C.  10xxxxxx

D.  110xxxxx

20.  Drag the steps in the DHCP process and place them in the correct

order on the right.

DHCPOffer

Drop Target A

DHCPDiscover Drop Target B

DHCPAck

Drop Target C

DHCPRequest Drop Target D

Chapter 4

Easy Subnetting

THE FOLLOWING ICND1 EXAM TOPICS ARE

COVERED IN THIS CHAPTER:

Network Fundamentals

1.8 Configure, verify, and troubleshoot IPv4 addressing and

subnetting

We’ll pick up right where we left off in the last

chapter and continue to explore the world of IP addressing. I’ll open this

chapter by telling you how to subnet an IP network—an indispensably

crucial skill that’s central to mastering networking in general!

Forewarned is forearmed, so prepare yourself because being able to

subnet quickly and accurately is pretty challenging and you’ll need time

to practice what you’ve learned to really nail it. So be patient and don’t

give up on this key aspect of networking until your skills are seriously

sharp. I’m not kidding—this chapter is so important you should really just

graft it into your brain!

So be ready because we’re going to hit the ground running and

thoroughly cover IP subnetting from the very start. And though I know

this will sound weird to you, you’ll be much better off if you just try to

forget everything you’ve learned about subnetting before reading this

chapter—especially if you’ve been to an official Cisco or Microsoft class! I

think these forms of special torture often do more harm than good and

sometimes even scare people away from networking completely. Those

that survive and persevere usually at least question the sanity of

continuing to study in this field. If this is you, relax, breathe, and know

that you’ll find that the way I tackle the issue of subnetting is relatively

painless because I’m going to show you a whole new, much easier method

to conquer this monster!

After working through this chapter, and I can’t say this enough, after

working through the extra study material at the end as well, you’ll be able

to tame the IP addressing/subnetting beast—just don’t give up! I promise

that you’ll be really glad you didn’t. It’s one of those things that once you

get it down, you’ll wonder why you used to think it was so hard!

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

.

Subnetting Basics

In Chapter 3, “Introduction to TCP/IP,” you learned how to define and

find the valid host ranges used in a Class A, Class B, and Class C network

address by turning the host bits all off and then all on. This is very good,

but here’s the catch: you were defining only one network, as shown in

Figure 4.1

.

FIGURE 4.1

One network

By now you know that having one large network is not a good thing

because the first three chapters you just read were veritably peppered

with me incessantly telling you that! But how would you fix the out-of-

control problem that

Figure 4.1

illustrates? Wouldn’t it be nice to be able

to break up that one, huge network address and create four manageable

networks from it? You betcha it would, but to make that happen, you

would need to apply the infamous trick of subnetting because it’s the best

way to break up a giant network into a bunch of smaller ones. Take a look

at

Figure 4.2

and see how this might look.

FIGURE 4.2

Multiple networks connected together

What are those 192.168.10.x addresses shown in the figure? Well that is

what this chapter will explain—how to make one network into many

networks!

Let’s take off from where we left in Chapter 3 and start working in the

host section (host bits) of a network address, where we can borrow bits to

create subnets.

How to Create Subnets

Creating subnetworks is essentially the act of taking bits from the host

portion of the address and reserving them to define the subnet address

instead. Clearly this will result in fewer bits being available for defining

your hosts, which is something you’ll always want to keep in mind.

Later in this chapter, I’ll guide you through the entire process of creating

subnets starting with Class C addresses. As always in networking, before

you actually implement anything, including subnetting, you must first

determine your current requirements and make sure to plan for future

conditions as well.

In this first section, we’ll be discussing classful routing, which

refers to the fact that all hosts (nodes) in the network are using the

exact same subnet mask. Later, when we move on to cover variable

length subnet masks (VLSMs), I’ll tell you all about classless routing,

which is an environment wherein each network segment can use a

different subnet mask.

To create a subnet, we’ll start by fulfilling these three steps:

1.  Determine the number of required network IDs:

One for each LAN subnet

One for each wide area network connection

2.  Determine the number of required host IDs per subnet:

One for each TCP/IP host

One for each router interface

3.  Based on the previous requirements, create the following:

A unique subnet mask for your entire network

A unique subnet ID for each physical segment

A range of host IDs for each subnet

Subnet Masks

For the subnet address scheme to work, every machine on the network

must know which part of the host address will be used as the subnet

address. This condition is met by assigning a subnet mask to each

machine. A subnet mask is a 32-bit value that allows the device that’s

receiving IP packets to distinguish the network ID portion of the IP

address from the host ID portion of the IP address. This 32-bit subnet

mask is composed of 1s and 0s, where the 1s represent the positions that

refer to the network subnet addresses.

Not all networks need subnets, and if not, it really means that they’re

using the default subnet mask, which is basically the same as saying that

a network doesn’t have a subnet address.

Table 4.1

shows the default

subnet masks for Classes A, B, and C.

Table 4.1

Default subnet mask

Class Format

Default Subnet Mask

A

network.node.node.node

255.0.0.0

B

network.network.node.node

255.255.0.0

C

network.network.network.node 255.255.255.0

Although you can use any mask in any way on an interface, typically it’s

not usually good to mess with the default masks. In other words, you

don’t want to make a Class B subnet mask read 255.0.0.0, and some hosts

won’t even let you type it in. But these days, most devices will. For a Class

A network, you wouldn’t change the first byte in a subnet mask because it

should read 255.0.0.0 at a minimum. Similarly, you wouldn’t assign

255.255.255.255 because this is all 1s, which is a broadcast address. A

Class B address starts with 255.255.0.0, and a Class C starts with

255.255.255.0, and for the CCNA especially, there is no reason to change

the defaults!

Understanding the Powers of 2

Powers of 2 are important to understand and memorize for use with

IP subnetting. Reviewing powers of 2, remember that when you see a

number noted with an exponent, it means you should multiply the

number by itself as many times as the upper number specifies. For

example, 2

3

is 2 x 2 x 2, which equals 8. Here’s a list of powers of 2 to

commit to memory:

2

1

= 2

2

2

= 4

2

3

= 8

2

4

= 16

2

5

= 32

2

6

= 64

2

7

= 128

2

8

= 256

2

9

= 512

2

10

= 1,024

2

11

= 2,048

2

12

= 4,096

2

13

= 8,192

2

14

= 16,384

Memorizing these powers of 2 is a good idea, but it’s not absolutely

necessary. Just remember that since you’re working with powers of 2,

each successive power of 2 is double the previous one.

It works like this—all you have to do to remember the value of 2

9

is to

first know that 2

8

= 256. Why? Because when you double 2 to the

eighth power (256), you get 2

9

(or 512). To determine the value of 2

10

,

simply start at 2

8

= 256, and then double it twice.

You can go the other way as well. If you needed to know what 2

6

is,

for example, you just cut 256 in half two times: once to reach 2

7

and

then one more time to reach 2

6

.

Classless Inter-Domain Routing (CIDR)

Another term you need to familiarize yourself with is Classless Inter-

Domain Routing (CIDR). It’s basically the method that Internet service

providers (ISPs) use to allocate a number of addresses to a company, a

home—their customers. They provide addresses in a certain block size,

something I’ll talk about in greater detail soon.

When you receive a block of addresses from an ISP, what you get will look

something like this: 192.168.10.32/28. This is telling you what your

subnet mask is. The slash notation (/) means how many bits are turned

on (1s). Obviously, the maximum could only be /32 because a byte is 8

bits and there are 4 bytes in an IP address: (4 × 8 = 32). But keep in mind

that regardless of the class of address, the largest subnet mask available

relevant to the Cisco exam objectives can only be a /30 because you’ve got

to keep at least 2 bits for host bits.

Take, for example, a Class A default subnet mask, which is 255.0.0.0.

This tells us that the first byte of the subnet mask is all ones (1s), or

11111111. When referring to a slash notation, you need to count all the 1

bits to figure out your mask. The 255.0.0.0 is considered a /8 because it

has 8 bits that are 1s—that is, 8 bits that are turned on.

A Class B default mask would be 255.255.0.0, which is a /16 because 16

bits are ones (1s): 11111111.11111111.00000000.00000000.

Table 4.2

has a listing of every available subnet mask and its equivalent

CIDR slash notation.

Table 4.2

CIDR values

Subnet Mask

CIDR Value

255.0.0.0

/8

255.128.0.0

/9

255.192.0.0

/10

255.224.0.0

/11

255.240.0.0

/12

255.248.0.0

/13

255.252.0.0

/14

255.254.0.0

/15

255.255.0.0

/16

255.255.128.0

/17

255.255.192.0

/18

255.255.224.0

/19

255.255.240.0

/20

255.255.248.0

/21

255.255.252.0

/22

255.255.254.0

/23

255.255.255.0

/24

255.255.255.128 /25

255.255.255.192 /26

255.255.255.224 /27

255.255.255.240 /28

255.255.255.248 /29

255.255.255.252 /30

The /8 through /15 can only be used with Class A network addresses. /16

through /23 can be used by Class A and B network addresses. /24

through /30 can be used by Class A, B, and C network addresses. This is a

big reason why most companies use Class A network addresses. Since

they can use all subnet masks, they get the maximum flexibility in

network design.

No, you cannot configure a Cisco router using this slash

format. But wouldn’t that be nice? Nevertheless, it’s really important

for you to know subnet masks in the slash notation (CIDR).

IP Subnet-Zero

Even though

ip subnet-zero

is not a new command, Cisco courseware

and Cisco exam objectives didn’t used to cover it. Know that Cisco

certainly covers it now! This command allows you to use the first and last

subnet in your network design. For instance, the Class C mask of

255.255.255.192 provides subnets 64 and 128, another facet of subnetting

that we’ll discuss more thoroughly later in this chapter. But with the

ip

subnet-zero

command, you now get to use subnets 0, 64, 128, and 192. It

may not seem like a lot, but this provides two more subnets for every

subnet mask we use.

Even though we don’t discuss the command-line interface (CLI) until

Chapter 6, “Cisco’s Internetworking Operating System (IOS),” it’s

important for you to be at least a little familiar with this command at this

point:

Router#

sh running-config

Building configuration...

Current configuration : 827 bytes

!

hostname Pod1R1

!

ip subnet-zero

!

This router output shows that the command

ip subnet-zero

is enabled on

the router. Cisco has turned this command on by default starting with

Cisco IOS version 12.x and now we’re running 15.x code.

When taking your Cisco exams, make sure you read very carefully to see if

Cisco is asking you not to use

ip subnet-zero

. There are actually

instances where this may happen.

Subnetting Class C Addresses

There are many different ways to subnet a network. The right way is the

way that works best for you. In a Class C address, only 8 bits are available

for defining the hosts. Remember that subnet bits start at the left and

move to the right, without skipping bits. This means that the only Class C

subnet masks can be the following:

Binary Decimal CIDR

---------------------------------------------------------

00000000 = 255.255.255.0 /24

10000000 = 255.255.255.128 /25

11000000 = 255.255.255.192 /26

11100000 = 255.255.255.224 /27

11110000 = 255.255.255.240 /28

11111000 = 255.255.255.248 /29

11111100 = 255.255.255.252 /30

We can’t use a /31 or /32 because, as I’ve said, we must have at least 2

host bits for assigning IP addresses to hosts. But this is only mostly true.

Certainly we can never use a /32 because that would mean zero host bits

available, yet Cisco has various forms of the IOS, as well as the new Cisco

Nexus switches operating system, that support the /31 mask. The /31 is

above the scope of the CCENT and CCNA objectives, so we won’t be

covering it in this book.

Coming up, I’m going to teach you that significantly less painful method

of subnetting I promised you at the beginning of this chapter, which

makes it ever so much easier to subnet larger numbers in a flash.

Excited? Good! Because I’m not kidding when I tell you that you

absolutely need to be able to subnet quickly and accurately to succeed in

the networking real world and on the exam too!