Easy Subnetting - Class C

I am going to teach you the easiest method of subnetting in the world.  There simply isn't any easier way to do subnetting than the way I'm about to show you.  I learned this method from Todd Lammle's CCNA Study Guide (a great buy for those of you who may be looking to write the CCNA and need some good study material)

We will start by working in the class C range for now and once you get that down, I will do some other posts for class B and class A as they are just slightly more difficult.

For a class C address we are subnetting in the fourth octet.

All you have to remember is that 256 - # = block size where # is the value in the subnet mask in the fourth octet.

The block size is the number that your subnets increment by!

Let's take a look:

192.168.10.34

255.255.255.224

1. the value in the subnet mask fourth octet is 224 so we do 256-224 = 32 

-now we now know our block size is 32 so we increment by 32 in the fourth octet to get the list of all the subnets

2. write out the subnets

-start at 192.168.10.0 and write out the subnets, we increment by 32 in the fourth octet so:

 192.168.10.32

                  <------------ our host 192.168.10.34 is in here

 192.168.10.64

 192.168.10.96

 ...and so on

3. now write out the hosts on our subnet

192.168.10.32 <--- our subnet address

192.168.10.33

192.168.10.34

192.168.10.35

...

...

192.168.10.61

192.168.10.62

192.168.10.63

192.168.10.64 <---- the next subnet address

4. now we just have to remember that the broadcast address is always one less than the next subnet address - lets look at our subnet again:

192.168.10.32 <--- our subnet address

192.168.10.33

192.168.10.34

192.168.10.35

...

...

192.168.10.61

192.168.10.62

192.168.10.63 <---- our broadcast address for the 192.168.10.32 subnet

192.168.10.64 <---- the next subnet address

5. and finally we just remember that the valid hosts are the addresses between our subnet address and our broadcast address

192.168.10.32 <--- our subnet address

192.168.10.33 <--- first valid host

192.168.10.34

192.168.10.35

...

...

192.168.10.61

192.168.10.62 <--- last valid host

192.168.10.63 <---- our broadcast address

192.168.10.64 <---- the next subnet address

It get's easier just watch - let's try another one.  Remember in class C we're in the 4th octet so don't even worry about the 192.168.10 part just add that after you get the number for the fourth octet

192.168.10.37

255.255.255.240

What's our block size? 256-240 = 16

What are our subnets? .0, .16, .32, .48, and so on

What subnet are we on? 192.168.10.32 (192.168.10.48 being the next subnet)

What's our broadcast address? 192.168.10.47 (one less than the next subnet address)

What are our valid host addresses? 192.168.10.33 to 192.168.10.46

Let's try one last one

192.168.10.17

255.255.255.252

What's our block size? 256-252 = 4

What are our subnets? .0, .4, .8, .12, .16, .20 and so on

What subnet are we on? 192.168.10.16 (192.168.10.20 being the next subnet address)

What is our broadcast address? 192.168.10.19 (one less than the next subnet address)

What are our valid hosts? 192.168.10.17 to 192.168.10.18

Conclusion: it all boils down to your block size.  

Find your block size (256-# in the subnet mask 4th octet)

Find your subnet

Find your broadcast (one less than the next subnet address)

Find your valid hosts (the addresses between your subnet address and your broadcast address)

To get even better at subnetting (and even be able to do it in your head) you should try to memorize the following table as it will allow you to quickly retrieve the information you need right off the top of your head.

Notice that mask + block size always = 256 ;)

128   192   224   240  248  252  254  255 mask
128    64    32    16    8    4    2    1 block size
/25   /26   /27   /28  /29  /30  /31  /32 slash notation