Ccna routing and Switching Complete Study Guide

Subnetting Class A Addresses

You don’t go about Class A subnetting any differently than Classes B and C, but there are 

24 bits to play with instead of the 16 in a Class B address and the 8 in a Class C address.

Let’s start by listing all the Class A masks:

255.0.0.0    (/8)

255.128.0.0  (/9)           255.255.240.0  (/20)

255.192.0.0  (/10)          255.255.248.0  (/21)

255.224.0.0  (/11)          255.255.252.0  (/22)

255.240.0.0  (/12)          255.255.254.0  (/23)

255.248.0.0  (/13)          255.255.255.0  (/24)

255.252.0.0  (/14)          255.255.255.128  (/25)

255.254.0.0  (/15)          255.255.255.192  (/26)

255.255.0.0  (/16)          255.255.255.224  (/27)

255.255.128.0  (/17)        255.255.255.240  (/28)

255.255.192.0  (/18)        255.255.255.248  (/29)

255.255.224.0  (/19)        255.255.255.252  (/30)

That’s it. You must leave at least 2 bits for defining hosts. I hope you can see the pattern 

by now. Remember, we’re going to do this the same way as a Class B or C subnet. It’s just 

that, again, we simply have more host bits and we just use the same subnet numbers we used 

with Class B and C, but we start using these numbers in the second octet. However, the rea-

son Class A addresses are so popular to implement is because they give the most flexibility. 

You can subnet in the second, third or fourth octet. I’ll show you this in the next examples.