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Chapter 3

  • IP Subnetting and Variable Length Subnet Masks (VLSMs)

Subnet

0

8

16

24

224

232

240

248

First host

1

9

17

25

225

233

241

249

Last host

6

14

22

30

230

238

246

254

Broadcast

7

15

23

31

231

239

247

255

Take a look at the following table. It shows some of the subnets (first four and last four only), valid hosts, and broadcast addresses for the Class C 255.255.255.248 mask:

Practice Example #5C: 255.255.255.252 (/30) Just a couple more: 192.168.10.0 = Network address 255.255.255.252 = Subnet mask Subnets? 64. Hosts? 2.

  • Valid subnets? 0, 4, 8, 12, etc., all the way to 252.

  • Broadcast address for each subnet? (always the number right before the next subnet)

  • Valid hosts? (the numbers between the subnet number and the broadcast address)

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 all hosts and router interfaces have the same subnet mask on each inter- face. You’ve read about classless routing where you can have different size 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.

If you use the 255.255.255.0 mask, then each network would have 254 hosts, but you only use two addresses with a WAN link! That is a waste of 252 hosts per subnet. If you use the 255.255.255.252 mask, then each subnet has only two hosts and you don’t waste precious addresses.

This is a really important subject, one that we’ll address in a lot more detail in the VLSM net- work design section later in this chapter.

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