Ccna routing and Switching Complete Study Guide

Subnet

0

4

8

12

…

240

244

248

252

First host

1

5

9

13

…

241

245

249

253

Last host

2

6

10

14

…

242

246

250

254

Broadcast

3

7

11

15

…

243

247

251

255

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Subnetting Basics 

151

Should we really use This mask That provides only Two hosts?

You are the network administrator for Acme Corporation in San Francisco, with dozens 

of WAN links connecting to your corporate office. Right now your network is a classful 

network, which means that the same subnet mask is on each host and router interface. 

You’ve read about classless routing, where you can have different sized masks, but don’t 

know what to use on your point-to-point WAN links. Is the 255.255.255.252 (/30) a helpful 

mask in this situation?

Yes, this is a very helpful mask in wide area networks and of course with any type of 

point-to-point link!

If you were to use the 255.255.255.0 mask in this situation, then each network would have 

254 hosts. But you use only 2 addresses with a WAN or point-to-point link, which is a 

waste of 252 hosts per subnet! If you use the 255.255.255.252 mask, then each subnet has 

only 2 hosts, and you don’t want to waste precious addresses. This is a really important 

subject, one that we’ll address in a lot more detail in the section on VLSM network design 

in the next chapter!