Senior Acquisitions Editor: Kenyon Brown Development Editor: Kim Wimpsett

FIGURE 5.17

IP address problem 2

A user in the Sales LAN can’t get to ServerB. You have the user run

through the four basic troubleshooting steps and find that the host can

communicate to the local network but not to the remote network. Find

and define the IP addressing problem.

If you went through the same steps used to solve the last problem, you

can see that first, the WAN link again provides the subnet mask to use—

/29, or 255.255.255.248. Assuming classful addressing, you need to

determine what the valid subnets, broadcast addresses, and valid host

ranges are to solve this problem.

The 248 mask is a block size of 8 (256 – 248 = 8, as discussed in Chapter

4), so the subnets both start and increment in multiples of 8. By looking

at the figure, you see that the Sales LAN is in the 24 subnet, the WAN is

in the 40 subnet, and the Marketing LAN is in the 80 subnet. Can you see

the problem yet? The valid host range for the Sales LAN is 25–30, and the

configuration appears correct. The valid host range for the WAN link is

41–46, and this also appears correct. The valid host range for the 80

subnet is 81–86, with a broadcast address of 87 because the next subnet

is 88. ServerB has been configured with the broadcast address of the

subnet.

Okay, now that you can figure out misconfigured IP addresses on hosts,

what do you do if a host doesn’t have an IP address and you need to

assign one? What you need to do is scrutinize the other hosts on the LAN

and figure out the network, mask, and default gateway. Let’s take a look

at a couple of examples of how to find and apply valid IP addresses to

hosts.

You need to assign a server and router IP addresses on a LAN. The subnet

assigned on that segment is 192.168.20.24/29. The router needs to be

assigned the first usable address and the server needs the last valid host

ID. What is the IP address, mask, and default gateway assigned to the

server?

To answer this, you must know that a /29 is a 255.255.255.248 mask,

which provides a block size of 8. The subnet is known as 24, the next

subnet in a block of 8 is 32, so the broadcast address of the 24 subnet is

31 and the valid host range is 25–30.

Server IP address: 192.168.20.30

Server mask: 255.255.255.248

Default gateway: 192.168.20.25 (router’s IP address)

Take a look at

Figure 5.18

and solve this problem.

FIGURE 5.18

Find the valid host #1

Look at the router’s IP address on Ethernet0. What IP address, subnet

mask, and valid host range could be assigned to the host?

The IP address of the router’s Ethernet0 is 192.168.10.33/27. As you

already know, a /27 is a 224 mask with a block size of 32. The router’s

interface is in the 32 subnet. The next subnet is 64, so that makes the

broadcast address of the 32 subnet 63 and the valid host range 33–62.

Host IP address: 192.168.10.34–62 (any address in the range except for

33, which is assigned to the router)

Mask: 255.255.255.224

Default gateway: 192.168.10.33

Figure 5.19

shows two routers with Ethernet configurations already

assigned. What are the host addresses and subnet masks of HostA and

HostB?

FIGURE 5.19

Find the valid host #2

Router A has an IP address of 192.168.10.65/26 and Router B has an IP

address of 192.168.10.33/28. What are the host configurations? Router A

Ethernet0 is in the 192.168.10.64 subnet and Router B Ethernet0 is in the

192.168.10.32 network.

Host A IP address: 192.168.10.66–126

Host A mask: 255.255.255.192

Host A default gateway: 192.168.10.65

Host B IP address: 192.168.10.34–46

Host B mask: 255.255.255.240

Host B default gateway: 192.168.10.33

Just a couple more examples before you can put this chapter behind you

—hang in there!

Figure 5.20

shows two routers. You need to configure the S0/0 interface

on RouterA. The IP address assigned to the serial link is 172.16.17.0/22.

What IP address can be assigned?

FIGURE 5.20

Find the valid host address #3

First, know that a /22 CIDR is 255.255.252.0, which makes a block size of

4 in the third octet. Since 17 is listed, the available range is 16.1 through

19.254, so in this example, the IP address S0/0 could be 172.16.18.255

since that’s within the range.

Okay, last one! You need to find a classful network address that has one

Class C network ID and you need to provide one usable subnet per city

while allowing enough usable host addresses for each city specified in

Figure 5.21

. What is your mask?

FIGURE 5.21

Find the valid subnet mask

Actually, this is probably the easiest thing you’ve done all day! I count 5

subnets needed, and the Wyoming office needs 16 users—always look for

the network that needs the most hosts! What block size is needed for the

Wyoming office? Your answer is 32. You can’t use a block size of 16

because you always have to subtract 2. What mask provides you with a

block size of 32? 224 is your answer because this provides 8 subnets, each

with 30 hosts.

You’re done—the diva has sung and the chicken has safely crossed the

road…whew! Time to take a break, but skip the shot and the beer if that’s

what you had in mind because you need to have your head straight to go

through the written lab and review questions next!

Summary

Again, if you got to this point without getting lost along the way a few

times, you’re awesome, but if you did get lost, don’t stress because most

people do! Just be patient with yourself and go back over the material

that tripped you up until it’s all crystal clear. You’ll get there!

This chapter provided you with keys to understanding the oh-so-very-

important topic of variable length subnet masks. You should also know

how to design and implement simple VLSM networks and be clear on

summarization as well.

And make sure you understand and memorize Cisco’s troubleshooting

methods. You must remember the four steps that Cisco recommends to

take when trying to narrow down exactly where a network and/or IP

addressing problem is and then know how to proceed systematically to fix

it. In addition, you should be able to find valid IP addresses and subnet

masks by looking at a network diagram.

Exam Essentials

Describe the benefits of variable length subnet masks (VLSMs).

VLSMs enable the creation of subnets of specific sizes and allow the

division of a classless network into smaller networks that do not need to

be equal in size. This makes use of the address space more efficient

because many times IP addresses are wasted with classful subnetting.

Understand the relationship between the subnet mask value

and the resulting block size and the allowable IP addresses in

each resulting subnet. The relationship between the classful network

being subdivided and the subnet mask used determines the number of

possible hosts or the block size. It also determines where each subnet

begins and ends and which IP addresses cannot be assigned to a host

within each subnet.

Describe the process of summarization or route aggregation

and its relationship to subnetting. Summarization is the combining

of subnets derived from a classful network for the purpose of advertising

a single route to neighboring routers instead of multiple routes, reducing

the size of routing tables and speeding the route process.

Calculate the summary mask that will advertise a single

network representing all subnets. The network address used to

advertise the summary address is always the first network address in the

block of subnets. The mask is the subnet mask value that yields the same

block size.

Remember the four diagnostic steps. The four simple steps that

Cisco recommends for troubleshooting are ping the loopback address,

ping the NIC, ping the default gateway, and ping the remote device.

Identify and mitigate an IP addressing problem. Once you go

through the four troubleshooting steps that Cisco recommends, you must

be able to determine the IP addressing problem by drawing out the

network and finding the valid and invalid hosts addressed in your

network.

Understand the troubleshooting tools that you can use from

your host and a Cisco router. The

ping 127.0.0.1

command tests

your local IP stack, and

tracert

is a Windows command to track the path

a packet takes through an internetwork to a destination. Cisco routers use

the command

traceroute

, or just

trace

for short. Don’t confuse the

Windows and Cisco commands. Although they produce the same output,

they don’t work from the same prompts. The command

ipconfig /all

will display your PC network configuration from a DOS prompt, and

arp

-a

(again from a DOS prompt) will display IP-to-MAC-address mapping

on a Windows PC.

Written Lab 5

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

the information and concepts contained within them fully dialed in:

Lab 5.1: Summarization Practice

You can find the answers to this lab in Appendix A, “Answers to Written

Labs.”

Lab 5.1: Summarization Practice

For each of the following sets of networks, determine the summary

address and the mask to be used that will summarize the subnets.

1.  192.168.1.0/24 through 192.168.12.0/24

2.  172.144.0.0 through 172.159.0.0

3.  192.168.32.0 through 192.168.63.0

4.  192.168.96.0 through 192.168.111.0

5.  66.66.0.0 through 66.66.15.0

6.  192.168.1.0 through 192.168.120.0

7.  172.16.1.0 through 172.16.7.0

8.  192.168.128.0 through 192.168.190.0

9.  53.60.96.0 through 53.60.127.0

10.  172.16.10.0 through 172.16.63.0

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.  On a VLSM network, which mask should you use on point-to-point

WAN links in order to reduce the waste of IP addresses?

A.  /27

B.  /28

C.  /29

D.  /30

E.  /31

2.  In the network shown in the diagram, how many computers could be

in Network B?

A.  6

B.  12

C.  14

D.  30

3.  In the following diagram, in order to have IP addressing that’s as

efficient as possible, which network should use a /29 mask?

A.  A

B.  B

C.  C

D.  D

4.  To use VLSM, what capability must the routing protocols in use

possess?

A.  Support for multicast

B.  Multiprotocol support

C.  Transmission of subnet mask information

D.  Support for unequal load balancing

5.  What summary address would cover all the networks shown and

advertise a single, efficient route to Router B that won’t advertise

more networks than needed?

A.  172.16.0.0/24

B.  172.16.1.0/24

C.  172.16.0.0/24

D.  172.16.0.0/20

E.  172.16.16.0/28

F.  172.16.0.0/27

6.  In the following diagram, what is the most likely reason the station

cannot ping outside of its network?

A.  The IP address is incorrect on interface E0 of the router.

B.  The default gateway address is incorrect on the station.

C.  The IP address on the station is incorrect.

D.  The router is malfunctioning.

7.  If a host is configured with an incorrect default gateway and all the

other computers and router are known to be configured correctly,

which of the following statements is TRUE?

A.  Host A cannot communicate with the router.

B.  Host A can communicate with other hosts in the same subnet.

C.  Host A can communicate with hosts in other subnets.

D.  Host A can communicate with no other systems.

8.  Which of the following troubleshooting steps, if completed

successfully, also confirms that the other steps will succeed as well?

A.  Ping a remote computer.

B.  Ping the loopback address.

C.  Ping the NIC.

D.  Ping the default gateway.

9.  When a ping to the local host IP address fails, what can you assume?

A.  The IP address of the local host is incorrect.

B.  The IP address of the remote host is incorrect.

C.  The NIC is not functional.

D.  The IP stack has failed to initialize.

10.  When a ping to the local host IP address succeeds but a ping to the

default gateway IP address fails, what can you rule out? (Choose all

that apply.)

A.  The IP address of the local host is incorrect.

B.  The IP address of the gateway is incorrect.

C.  The NIC is not functional.

D.  The IP stack has failed to initialize.

11.  Which of the networks in the diagram could use a /29 mask?

A.  Corporate

B.  LA

C.  SF

D.  NY

E.  None

12.  What network service is the most likely problem if you can ping a

computer by IP address but not by name?

A.  DNS

B.  DHCP

C.  ARP

D.  ICMP

13.  When you issue the

ping

command, what protocol are you using?

A.  DNS

B.  DHCP

C.  ARP

D.  ICMP

14.  Which of the following commands displays the networks traversed on

a path to a network destination?

A.

ping

B.

traceroute

C.

pingroute

D.

pathroute

15.  What command generated the output shown below?

Reply from 172.16.10.2: bytes=32 time<1ms TTL=128

Reply from 172.16.10.2: bytes=32 time<1ms TTL=128

Reply from 172.16.10.2: bytes=32 time<1ms TTL=128

Reply from 172.16.10.2: bytes=32 time<1ms TTL=128

A.

traceroute

B.

show ip route

C.

ping

D.

pathping

16.  In the work area, match the command to its function on the right.

17.  Which of the following network addresses correctly summarizes the

three networks shown below efficiently?

10.0.0.0/16

10.1.0.0/16

10.2.0.0/16

A.  10.0.0.0/15

B.  10.1.0.0/8

C.  10.0.0.0/14

D.  10.0.0.8/16

18.  What command displays the ARP table on a Cisco router?

A.

show ip arp

B.

traceroute

C.

arp -a

D.

tracert

19.   What switch must be added to the

ipconfig

command on a PC to

verify DNS configuration?

A.

/dns

B.

-dns

C.

/all

D.

showall

20.  Which of the following is the best summarization of the following

networks: 192.168.128.0 through 192.168.159.0?

A.  192.168.0.0/24

B.  192.168.128.0/16

C.  192.168.128.0/19

D.  192.168.128.0/20

Chapter 6

Cisco’s Internetworking Operating System (IOS)

The following ICND1 exam topics are covered in

this chapter:

2.0 LAN Switching Technologies

2.3 Troubleshoot interface and cable issues (collisions,

errors, duplex, speed)

5.0 Infrastructure Management

5.3 Configure and verify initial device configuration

5.4 Configure, verify, and troubleshoot basic device

hardening

5.4.a Local authentication

5.4.b Secure password

5.4.c Access to device

5.4.c.(i) Voice

5.4.c.(ii) Video

5.4.c. (iii) Data

5.4.d Source address Telnet/SSH

5.4.e Login banner

5.6 Use Cisco IOS tools to troubleshoot and resolve

problems

5.6.aPing and traceroute with extended option

5.6.bTerminal monitor

5.6.c Log events

It’s time to introduce you to the Cisco

Internetwork Operating System (IOS). The IOS is what runs Cisco routers

as well as Cisco’s switches, and it’s also what we use to configure these

devices.

So that’s what you’re going to learn about in this chapter. I’m going to

show you how to configure a Cisco IOS device using the Cisco IOS

command-line interface (CLI). Once proficient with this interface, you’ll

be able to configure hostnames, banners, passwords, and more as well as

troubleshoot skillfully using the Cisco IOS.

We’ll also begin the journey to mastering the basics of router and switch

configurations plus command verifications in this chapter.

I’ll start with a basic IOS switch to begin building the network we’ll use

throughout this book for configuration examples. Don’t forget—I’ll be

using both switches and routers throughout this chapter, and we

configure these devices pretty much the same way. Things diverge when

we get to the interfaces where the differences between the two become

key, so pay attention closely when we get to that point!

Just as it was with preceding chapters, the fundamentals presented in

this chapter are important building blocks to have solidly in place before

moving on to the more advanced material coming up in the next ones.

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

.

The IOS User Interface

The Cisco Internetwork Operating System (IOS) is the kernel of Cisco

routers as well as all current Catalyst switches. In case you didn’t know, a

kernel is the elemental, indispensable part of an operating system that

allocates resources and manages tasks like low-level hardware interfaces

and security.

Coming up, I’ll show you the Cisco IOS and how to configure a Cisco

switch using the command-line interface (CLI). By using the CLI, we can

provide access to a Cisco device and provide voice, video, and data

service. . . . The configurations you’ll see in this chapter are exactly the

same as they are on a Cisco router.

Cisco IOS

The Cisco IOS is a proprietary kernel that provides routing, switching,

internetworking, and telecommunications features. The first IOS was

written by William Yeager in 1986 and enabled networked applications. It

runs on most Cisco routers as well as a growing number of Cisco Catalyst

switches, like the Catalyst 2960 and 3560 series switches used in this

book. And it’s an essential for the Cisco exam objectives!

Here’s a short list of some important things that the Cisco router IOS

software is responsible for:

Carrying network protocols and functions

Connecting high-speed traffic between devices

Adding security to control access and stopping unauthorized network

use

Providing scalability for ease of network growth and redundancy

Supplying network reliability for connecting to network resources

You can access the Cisco IOS through the console port of a router or

switch, from a modem into the auxiliary (or aux) port on a router, or even

through Telnet and Secure Shell (SSH). Access to the IOS command line

is called an EXEC session.

Connecting to a Cisco IOS Device

We connect to a Cisco device to configure it, verify its configuration, and

check statistics, and although there are different approaches to this, the

first place you would usually connect to is the console port. The console

port is usually an RJ45, 8-pin modular connection located at the back of

the device, and there may or may not be a password set on it by default.

Look back into Chapter 2, “Ethernet Networking and Data

Encapsulation,” to review how to configure a PC and enable it to

connect to a router console port.

You can also connect to a Cisco router through an auxiliary port, which is

really the same thing as a console port, so it follows that you can use it as

one. The main difference with an auxiliary port is that it also allows you

to configure modem commands so that a modem can be connected to the

router. This is a cool feature because it lets you dial up a remote router

and attach to the auxiliary port if the router is down and you need to

configure it remotely, out-of-band. One of the differences between Cisco

routers and switches is that switches do not have an auxiliary port.

The third way to connect to a Cisco device is in-band, through the

program Telnet or Secure Shell (SSH). In-band means configuring the

device via the network, the opposite of out-of-band. We covered Telnet

and SSH in Chapter 3, “Introduction to TCP/IP,” and in this chapter, I’ll

show you how to configure access to both of these protocols on a Cisco

device.

Figure 6.1

shows an illustration of a Cisco 2960 switch. Really focus in on

all the different kinds of interfaces and connections! On the right side is

the 10/100/1000 uplink. You can use either the UTP port or the fiber

port, but not both at the same time.