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Subnetting Class C Addresses

125

Subnet

0.0

1.0

2.0

3.0

...

254.0

255.0

First host

0.1

1.1

2.1

3.1

...

254.1

255.1

Last host

0.254

1.254

2.254

3.254

...

254.254

255.254

Broadcast

0.255

1.255

2.255

3.255

...

254.255

255.255

The following table shows the first four subnets and the last two, valid hosts, and broadcast addresses in a Class B 255.255.255.0 mask:

Practice Example #5B: 255.255.255.128 (/25)

This is one of the hardest subnet masks you can play with, though. And worse, it actually is a really good subnet to use in production because it creates over 500 subnets with 126 hosts for each subnet—a nice mixture. So, don’t skip over it!

172.16.0.0 = Network address

        • 255.255.255.128

          = Subnet mask

  • Subnets? 29 = 512.

  • Hosts? 27 – 2 = 126.

  • Valid subnets? Okay, now for the tricky part. 256 – 255 = 1. 0, 1, 2, 3, etc., for the third octet. But you can’t forget the one subnet bit used in the fourth octet. Remember when I showed you how to figure one subnet bit with a Class C mask? You figure this the same way. (Now you know why I showed you the 1-bit subnet mask in the Class C section—to make this part easier.) You actually get two subnets for each third octet value, hence the 512 subnets. For example, if the third octet is showing subnet 3, the two subnets would actually be 3.0 and 3.128.

  • Broadcast address for each subnet?

  • Valid hosts?

The following table shows how you can create subnets, valid hosts, and broadcast addresses using the Class B 255.255.255.128 subnet mask (the first eight subnets are shown, and then the last two subnets):

Subnet 0.0

0.128 1.0

1.128 2.0

2.128

3.0

3.128

... 255.0

255.128

First 0.1

0.129 1.1

1.129 2.1

2.129

3.1

3.129

... 255.1

255.129

Last host

0.126 0.254 1.126 1.254 2.126 2.254

3.126 3.254

... 255.126 255.254

Broad- 0.127 0.255 1.127 1.255 2.127 2.255 cast

3.127 3.255

... 255.127 255.255

host

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