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Chapter 3

  • IP Subnetting and Variable Length Subnet Masks (VLSMs)

Practice Example #3B: 255.255.254.0 (/23) 172.16.0.0 = Network address 255.255.254.0 = Subnet mask Subnets? 27 = 128. Hosts? 29 – 2 = 510. Valid subnets? 256 – 254 = 0, 2, 4, 6, 8, etc., up to 254. Broadcast address for each subnet? Valid hosts?

The following table shows the first five subnets, valid hosts, and broadcast addresses in a Class B 255.255.254.0 mask:

Subnet

0.0

2.0

4.0

6.0

8.0

First host

0.1

2.1

4.1

6.1

8.1

Last host

1.254

3.254

5.254

7.254

9.254

Broadcast

1.255

3.255

5.255

7.255

9.255

In your studies, remember that it’s very important for you to know your Class B /23 mask, and how many subnets and hosts it provides!

Practice Example #4B: 255.255.255.0 (/24)

Contrary to popular belief, 255.255.255.0 used with a Class B network address is not called a Class B network with a Class C subnet mask. It’s amazing how many people see this mask used in a Class B network and think it’s a Class C subnet mask. This is a Class B subnet mask with 8 bits of subnetting—it’s considerably different from a Class C mask. Subnetting this address is fairly simple:

172.16.0.0 = Network address 255.255.255.0 = Subnet mask Subnets? 28 = 256. Hosts? 28 – 2 = 254. Valid subnets? 256 – 255 = 1. 0, 1, 2, 3, etc. all the way to 255. Broadcast address for each subnet? Valid hosts?

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