By now you should feel able to work out basic subnets and determine which subnet a host is in.  You should also be able to work out the first host, last host and broadcast address of an IP address.

I do have a very special mission for you now.  You may laugh at me or just dismiss me but please do what I ask of you and I promise it will make a big difference.

I want you to teach somebody how to subnet!  Hello...is anybody there...where did you just go to?  Get back out from under your desk, I can see you.  Just find some kind and gentle person, explain to them that you have just learned something new and want to practise by showing them.  The trick of course is to work out the answer and how you did it BEFORE showing them.  Pick a nice easy example and away we go.

Onto the lesson.  I will show you how to do a very basic network design using the chart.

Your manager comes to you with the network address 192.168.1.0 and says that he needs it breaking down so he has at least 6 subnets and each subnet needs 25 hosts.  The office is having a big restructure and he wants each team to have their own subnet for security reasons.

Grab you pen and away we go....

You know that it is a Class C network so you can only steal bits from the last octet and the octet has 8 bits.  Tick the lower powers of two column on the Subnetting Secrets cheat sheet.  Tick until you get to the number that gives you the 6 subnets.

2 not enough
4 not enough
8 yes, as close as you can get to 6

 

2(-2)
4
8
16
32
64
128
256
etc ..

Now tick three numbers down on the upper subnet numbers.

 

The number you ticked was 224 so the answer is 255.255.255.224 will give you the 6 subnets he wants (with 2 to spare).

I can see you shaking your head, but that really is it!  You have stolen 3 bits for the subnet so you have 5 bits left for the hosts on the subnets.  Tick down 5 places on the powers of two column and remember to take two away from that to cover the subnet number and the broadcast number.
 

2(-2)
4
8
16
32
64
128
256
etc ..

Ticking down 5 places gives us 30 hosts (32 -2).  So the subnet mask 255.255.255.224 gives your boss what he needs as close as the maths will allow.  8 subnets each having 30 hosts.

There are more design examples in the two e-books and videos in Subnetting Secrets.  There are three full network design videos for you to enjoy as well.

http://www.subnetting-secrets.com/buy_subnetting_secrets.html

Or it is given away to all members of http://www.howtonetwork.net which is a unique Cisco CCNA online training site I have created.

Last lesson tomorrow (hooray).

  Or call us on the toll free numbers at the top.