ICND1 Subnetting confusion
I was creating some of my own practice subnetting questions and I'm kind of confused here. I might just be making a mistake, but when I follow the directions as lined out in video 19 i am coming up with some strange results.
For instance, lets say I'm subnetting a class c network 126.96.36.199 and want to create 8 networks.
So I line out my network and host bits like so
128 - 64 - 32 - 16 - 8 - 4 - 2 - 1
0 - 0 - 0 - 0 - [1 - 0 - 0 - 0] - four bit positions to make 8
so I represent these four bits in the binary and convert
11111111 11111111 11111111 11110000
256-240 = 16 for my increment, but this gives me 16 networks instead of the 8 that I need.
The same thing happens when I want to create a subnet with 4 networks. If I use network sizes that differ from my bit representations (10, 9, 7 etc.), my math comes out right. Is this just a quirk in the way the math is done or am I doing something wrong?
Thanks for any help!
Your not doing anything wrong, keep watching the videos, eventually Jeremy will explain about exceptions to the rule. If I remember rightly he calls them GREAT EXCEPTIONS.
Basically with networks required certain numbers; these being 2,4,8,16,32,64 etc if you req any of these numbers just -1 network.
So 8 network required calculate for 7.
As for hosts req the exception numbers are;3,7,15,31,63 etc.
This time just +1; e.g. 7 Hosts req calculate for 8.
You will find your above calculations work. But the last octect (.240) would end up being .248
All the best
The issue is that your using four bits for 8 networks when you only need to use three bits.
I believe you are getting confused with the 8 bits used in each octet (128 - 64 - 32 - 16 - 8 - 4 - 2 - 1) - as you can see, 8 is in the fourth position along. However, the actual network formula starts with 2, then goes upwards. In other words, it is:
128 - 64 - 32 - 16 - 8 - 4 - 2
As you can see, 8 is now in the third position along. If you re do your subnetting using this method instead of the one you are currently using, you'll find it works every time.