CISCO - CCNA 200-120 Exam – Subnetting The 5 golden rules for Subnetting: 1- IP addresses within a given subnetwork must be delimited within the boundary of the subnet mask. 2- All stations within a given subnetwork must share the same subnetwork mask. 3- All stations within a given subnetwork must be able to receive MAC layer broadcast frames from any device in the subnetwork. 4- If the source and destination addresses of two stations do not fall into the same subnetwork, communication between them can only occur via the default router. 5- Only a router can resolve addresses between different subnetworks.

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EASY SUBNETTING
Here’s A Way Of Learning Subnetting That I’ve Learned:
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 Network Bits 16 Host Bits ; range is 128-191 in the first octet
Class C Address: - 24 Network Bits 8 Host Bits ; range is 192-223 in the first octet
Now, remember this GOLDEN RULE:
1 - 128
2 - 64
3 - 32
4 - 16
5 - 8
6 - 4
7 - 2
8 - 1
Explanation:
First part is The Number of Host Bits
The Second Part Is The “Increment”.
As long as you remember this rule, you should be able to answer most subnetting questions in flash.
Let’s take the third line, 3 – 32
Example: 192.168.23.46/27, what subnet should this be in?
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:
192.168.23.0
192.168.23.32
192.168.23.64 and so on…
the address given to us is 192.168.23.46, so that’ll fall in 192.168.23.32 subnet - as 46 lies between 32 and 64.

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Here’s Another Example: 192.168.1.50/28
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:
192.168.1.0
192.168.1.16
192.168.1.32
192.168.1.48
192.168.1.64
And so on…
The address has the last octet value of 50, which lies in the range 48 – 64
So the address is in the 192.168.1.48 subnet.
Try This A Class B Example: What subnet does host 172.28.12.167/20 belong to?
It’s 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:
172.28.0.0
172.28.16.0
172.28.32.0
And so on..
Third octet in this example is 12
So it should lie in the first address (between 0 and 16): 172.28.0.0
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.

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Example: 192.168.23.46 255.255.255.240
All you do is:
256-240 = 16.
16 is your increment value!
Example: 172.15.23.187 255.255.248.0
All you do is: 256-248=8. 8 is your increment value and
Here’s what the subnets should look like:
172.15.0.0
172.15.8.0
172.15.16.0
And so on..