Ccna routing and Switching Complete Study Guide

172.16.17.0/22

Router A

S0/0

S0/0

Router B

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 net-

work 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?

f I g u r e   5 . 21     Find the valid subnet mask

NY

7 users

Corporate

7 users

SF

13 users

LA

15 users

WY

16 users

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 awe-

some, but if you did get lost, don’t stress because most people do! Just be patient with your-

self 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.

---

Exam Essentials 

197

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