An easier way of subnetting

Here’s a way of learning subnetting that I’ve come up with:
You should know this:
Class A Address: 8 network bits - 24 host bits; range is 0-127 in the first octet
Class B Address: 16 net bits - 16 host bits; range is 128-191 in the first oct
Class C Address: 24 net bits - 8 host bits; range is 192-223 in the first oct
Now, remember this golden rule:
1 - 128
2 - 64
3 - 32
4 - 16
5 - 8
6 - 4
7 - 2
8 - 1
As long as you remember this (very easy) rule, you should be able to answer most subnetting questions in flash.

First part is the number of Host bits and the second part is the ‘increment’.
Let’s take the third line, 3 - 32.
This means, if the number of host bits is 3, you should increment the subnet by 32.
Let me elaborate: - what subnet should this be in?
This is all you need to do:
27-24 = 3 (since this is a class C address, you subtract 24 network bits from 27 and that gives you 3 host bits)
3 should correspond to 32 (according to the golden rule I gave you above)
that means increment is 32
so the subnet range should be:
and so on…
the address given to us is, so that’ll fall in subnet (as 46 lies between 32 and 64)
Here’s another example.
Again, class C address. So this is what you do: 28-24=4
Golden rule: 4 corresponds to 16, so 16 should be the ‘increment’.
So here we go:
and so on…
our address has the last octet value of 50, which lies in the range 48 - 64, so the address is in the subnet.
Let’s try a Class B example now:
What subnet does host belong to?

Its a class B address so this is what we do: 20 - 16 = 4
4 should correspond to 32 going by the golden rule, so increment value is 32 here.
Since it’s a class B address, here’s how we can break the bits into Network and Host bits:
Typical Class B address: 8 Net bits - 8 Net bits (16) - 8 Host bits - 8 Host bits (16)
This example: 8 Net bits - 8 Net bits - 4 Host bits - 0 Host bits (we only have 4 Host bits, remember? 20-16 = 4)
So the increment would be in 3rd octet.
Here we go: and so on..
Third octet in this example is 12, so it should lie in the first address (between 0 and 16):

If you’re preparing for a CCNA exam, this rule should help a lot, as you can easily remember it and after some practice, these values do get embedded in your brain. Saves you a lot of time.
If the Subnet mask has been provided in dotted format, just subtract the first non-zero octet value from right by 256 and that’ll give you the ‘increment’ value.
All you do is: 256-240 = 16. 16 is your increment value!
Another example,
All you do is: 256-248=8. 8 is your increment value and here’s what the subnets should look like: and so on..
Hope this helps. Cheers!

No comments:

Powered by Blogger.