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Chapter 3

  • IP Subnetting and Variable Length Subnet Masks (VLSMs)

  • What are the valid hosts? These are the numbers between the subnet and broadcast address. The easiest way to find the hosts is to write out the subnet address and the broadcast address. This way, the valid hosts are obvious. The following table shows the 0, 64, 128, and 192 subnets, the valid host ranges of each, and the broadcast address of each subnet:

The subnets (do this first) Our first host (perform host addressing last) Our last host The broadcast address (do this second)

0 1 62 63

64

128

192

65

129

193

126

190

254

127

191

255

See? We really did come up with the same answers as when we did it the binary way, and this way is so much easier because you never have to do any binary-to-decimal conversions! About now, you might be thinking that it’s not easier than the first method I showed you. And I’ll admit, for the first subnet with only 2 subnet bits—you’re right, it isn’t that much easier. But remember, we’re going after the gold: being able to subnet in your head. And to do that, you need one thing: practice!

Practice Example #2C: 255.255.255.224 (/27) This time, we’ll subnet the network address 192.168.10.0 and subnet mask 255.255.255.224. 192.168.10.0 = Network address

        • 255.255.255.224

          = Subnet mask

  • How many subnets? 224 is 11100000, so our equation would be 23 = 8.

  • How many hosts? 25 – 2 = 30.

  • What are the valid subnets? 256 – 224 = 32. We just start at zero and count to the subnet mask value in blocks (increments) of 32: 0, 32, 64, 96, 128, 160, 192, 224.

  • What’s the broadcast address for each subnet (always the number right before the next subnet)?

  • What are the valid hosts (the numbers between the subnet number and the broadcast address)? To answer questions 4 and 5, first just write out the subnets, then write out the broadcast

addresses—the number right before the next subnet. Lastly, fill in the host addresses. The fol- lowing table gives you all the subnets for the 255.255.255.224 Class C subnet mask:

The subnet address

0

32

64

96

128

160

192

224

The first valid host

1

33

65

97

129

161

193

225

The last valid host

30

62

94

126

158

190

222

254

The broadcast address

31

63

95

127

159

191

223

255

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