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Chapter 3

  • IP Subnetting and Variable Length Subnet Masks (VLSMs)

255.255.255.0

(/24)

255.255.255.128

(/25)

255.255.255.192

(/26)

255.255.255.224

(/27)

255.255.255.240

(/28)

255.255.255.248

(/29)

255.255.255.252

(/30)

Subnetting Class B Addresses

Before we dive into this, let’s look at all the possible Class B subnet masks first. Notice that we have a lot more possible subnet masks than we do with a Class C network address:

255.255.128.0

(/17)

255.255.192.0

(/18)

255.255.224.0

(/19)

255.255.240.0

(/20)

255.255.248.0

(/21)

255.255.252.0

(/22)

255.255.254.0

(/23)

We know the Class B network address has 16 bits available for host addressing. This means we can use up to 14 bits for subnetting (because we have to leave at least 2 bits for host addressing).

By the way, do you notice anything interesting about that list of subnet val- ues—a pattern, maybe? Ah ha! That’s exactly why I had you memorize the binary-to-decimal numbers at the beginning of this section. Since subnet mask bits start on the left, move to the right, and can’t skip bits, the numbers are always the same regardless of the class of address. Memorize this pattern.

The process of subnetting a Class B network is pretty much the same as it is for a Class C, except that you just have more host bits. Use the same subnet numbers for the third octet with Class B that you used for the fourth octet with Class C, but add a zero to the network portion and a 255 to the broadcast section in the fourth octet. The following table shows you an example host range of two subnets used in a Class B subnet:

First subnet

16.0

32.0

Second subnet

16.255

32.255

Just add the valid hosts between the numbers, and you’re set!

This above example is only true until you get up to /24. After that, it’s numeri- cally exactly like Class C.

Subnetting Practice Examples: Class B Addresses

This section will give you an opportunity to practice subnetting Class B addresses.

Practice Example #1B: 255.255.192.0 (/18) 172.16.0.0 = Network address 255.255.192.0 = Subnet mask

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