Ccna routing and Switching Complete Study Guide

Lab A

Lab B

192.168.1.33

Default gateway:

192.168.1.62

Fa0/0 192.168.1.95

192.168.1.100/27

DCE

S0/0

Fa0/0 192.168.1.62

S0/0

192.168.1.97/27

ServerA

192.168.1.66

Default gateway:

192.168.1.95

ServerB

192.168.1.65

Default gateway:

192.168.1.95

Sales

Marketing

---

Troubleshooting IP Addressing 

193

A user in the sales department calls and tells you that she can’t get to ServerA in the mar-

keting department. You ask her if she can get to ServerB in the marketing department, but 

she doesn’t know because she doesn’t have rights to log on to that server. What do you do?

First, guide your user through the four troubleshooting steps you learned in the pre-

ceding section. Okay—let’s say steps 1 through 3 work but step 4 fails. By looking at the 

figure, can you determine the problem? Look for clues in the network drawing. First, the 

WAN link between the Lab A router and the Lab B router shows the mask as a /27. You 

should already know that this mask is 255.255.255.224 and determine that all networks 

are using this mask. The network address is 192.168.1.0. What are our valid subnets and 

hosts? 256 – 224 = 32, so this makes our subnets 0, 32, 64, 96, 128, etc. So, by looking at 

the figure, you can see that subnet 32 is being used by the sales department. The WAN link 

is using subnet 96, and the marketing department is using subnet 64.

Now you’ve got to establish what the valid host ranges are for each subnet. From what 

you learned at the beginning of this chapter, you should now be able to easily determine 

the subnet address, broadcast addresses, and valid host ranges. The valid hosts for the 

Sales LAN are 33 through 62, and the broadcast address is 63 because the next subnet is 

64, right? For the Marketing LAN, the valid hosts are 65 through 94 (broadcast 95), and 

for the WAN link, 97 through 126 (broadcast 127). By closely examining the figure, you 

can determine that the default gateway on the Lab B router is incorrect. That address is the 

broadcast address for subnet 64, so there’s no way it could be a valid host!

If you tried to configure that address on the Lab B router interface, you’d 

receive a 

bad mask

 error. Cisco routers don’t let you type in subnet and 

broadcast addresses as valid hosts!

Did you get all that? Let’s try another one to make sure. Figure 5.17 shows a network 

problem.

f I g u r e   5 .17     IP address problem 2