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Chapter 3

  • IP Subnetting and Variable Length Subnet Masks (VLSMs)

As with the /23 mask, it’s also really important for you to know your Class B / 25 mask and how many subnets and hosts it provides!

Practice Example #6B: 255.255.255.192 (/26)

Now, this is where Class B subnetting gets easy. Since the third octet has a 255 in the mask sec- tion, whatever number is listed in the third octet is a subnet number. However, now that we have a subnet number in the fourth octet, we can subnet this octet just like we did with Class C subnetting. Let’s try it out:

172.16.0.0 = Network address

        • 255.255.255.192

          = Subnet mask

  • Subnets? 210 = 1024.

  • Hosts? 26 – 2 = 62.

  • Valid subnets? 256 – 192 = 64. The subnets are shown in the following table. Do these numbers look familiar?

  • Broadcast address for each subnet?

  • Valid hosts?

The following table shows the first eight subnet ranges, valid hosts, and broadcast addresses:

Subnet

0.0

0.64

0.128

0.192

1.0

1.64

1.128

1.192

First host

0.1

0.65

0.129

0.193

1.1

1.65

1.129

1.193

Last host

0.62

0.126

0.190

0.254

1.62

1.126

1.190

1.254

Broadcast

0.63

0.127

0.191

0.255

1.63

1.127

1.191

1.255

Notice that for each subnet value in the third octet, you get subnets 0, 64, 128, and 192 in the fourth octet.

Practice Example #7B: 255.255.255.224 (/27)

This is done the same way as the preceding subnet mask, except that we just have more subnets and fewer hosts per subnet available.

172.16.0.0 = Network address 255.255.255.224 = Subnet mask Subnets? 211 = 2048. Hosts? 25 – 2 = 30. Valid subnets? 256 – 224 = 32. 0, 32, 64, 96, 128, 160, 192, 224. Broadcast address for each subnet? Valid hosts?

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