Ccna routing and Switching Complete Study Guide

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:

255.255.0.0    (/16)

255.255.128.0  (/17)      255.255.255.0    (/24)

255.255.192.0  (/18)      255.255.255.128  (/25)

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Subnetting Basics 

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255.255.224.0  (/19)      255.255.255.192  (/26)

255.255.240.0  (/20)      255.255.255.224  (/27)

255.255.248.0  (/21)      255.255.255.240  (/28)

255.255.252.0  (/22)      255.255.255.248  (/29)

255.255.254.0  (/23)      255.255.255.252  (/30)

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 need to leave at least 2 bits for 

host addressing. Using a /16 means you are not subnetting with Class B, but it is a mask 

you can use!

By the way, do you notice anything interesting about that list of subnet 

values—a pattern, maybe? Ah ha! That’s exactly why I had you memorize 

the binary-to-decimal numbers earlier in Chapter 2, “Ethernet Network-

ing and Data Encapsulation.” Since subnet mask bits start on the left and 

move to the right and bits can’t be skipped, the numbers are always the 

same regardless of the class of address. If you haven’t already, 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 have more host bits and you start in the third octet.

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 broad-

cast section in the fourth octet. The following table shows you an example host range of 

two subnets used in a Class B 240 (/20) subnet mask: