Ccna routing and Switching Complete Study Guide

R2

172.1.4.0/25

172.1.4.128/25

172.1.5.0/24

172.1.6.0/24

172.1.7.0/24

R1

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Troubleshooting IP Addressing 

189

I’ll be honest with you—this is a much harder question than the one in Figure 5.13, so 

you’re going to have to look carefully to see the answer. A good approach here would be to 

write down all the networks and see if you can find anything in common with all of them:

 

■

172.1.4.128/25

 

■

172.1.7.0/24

 

■

172.1.6.0/24

 

■

172.1.5.0/24

 

■

172.1.4.0/25

Do you see an octet that looks interesting to you? I do. It’s the third octet. 4, 5, 6, 7, and 

yes, it’s a block size of 4. So you can summarize 172.1.4.0 using a mask of 255.255.252.0, 

meaning you would use a block size of 4 in the third octet. The IP addresses forwarded 

with this summary would be 172.1.4.1 through 172.1.7.254.

To summarize the summarization section, if you’ve nailed down your block sizes, then 

finding and applying summary addresses and masks is a relatively straightforward task. But 

you’re going to get bogged down pretty quickly if you don’t know what a /20 is or if you 

can’t count by 16s!

Troubleshooting IP Addressing

Because running into trouble now and then in networking is a given, being able to trouble-

shoot IP addressing is clearly a vital skill. I’m not being negative here—just realistic. The 

positive side to this is that if you’re the one equipped with the tools to diagnose and clear 

up the inevitable trouble, you get to be the hero when you save the day! Even better? You 

can usually fix an IP network regardless of whether you’re on site or at home!

So this is where I’m going to show you the “Cisco way” of troubleshooting IP address-

ing. Let’s use Figure 5.15 as an example of your basic IP trouble—poor Sally can’t log in to 

the Windows server. Do you deal with this by calling the Microsoft team to tell them their 

server is a pile of junk and causing all your problems? Though tempting, a better approach 

is to first double-check and verify your network instead.

f I g u r e   5 .15     Basic IP troubleshooting

E0

172.16.10.1

Sally

172.16.10.2

Server

172.16.20.2

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190

 

Chapter 5 

■

  VLSMs, Summarization, and Troubleshooting TCP/IP 

Okay, let’s get started by going through the troubleshooting steps that Cisco recom-

mends. They’re pretty simple, but important nonetheless. Pretend you’re at a customer host 

and they’re complaining that they can’t communicate to a server that just happens to be on 

a remote network. Here are the four troubleshooting steps Cisco recommends:

1.

  Open a Command window and ping 127.0.0.1. This is the diagnostic, or loopback, 

address, and if you get a successful ping, your IP stack is considered initialized. If it 

fails, then you have an IP stack failure and need to reinstall TCP/IP on the host.

C:\>