You think you have to be Superbrain to understand it?
You are wrong!
Here the step-by-step course.
After reading and some self-training, you should be able to fix Subnetting-Questions in CCNA Exam
without any problems in a snatch.
Relax!
What is a Subnetmask?
With Subnetmasks, we can divide an IP-Address in network-part and in host-part.
A given IP-Network can be divided in smaller parts. Each of this smaller parts is called a "Subnet".
If we for example have the network
192.168.10.0 255.255.255.0
We have here ONE Class C - network, with 253 useable IPs for Client-PCs.
The useable IP Range of this network is
192.168.10.1 - 192.168.10.254
The very last IP of each Subnet is called Broadcast-Address.
This address is in that example 192.168.10.255 and its NOT useable for host-pcs.
If we want to divide this network in two parts, we must use subnetting.
With Subnetmask 255.255.255.128 we would divide the network in two parts.
192.168.10.1 - 192.168.10.127
192.168.10.128 - 192.168.10.255
So in this example, BEFORE we had one big Network.
With the change of the Subnet mask we did divide it in two smaller networks.
First with Subnet mask 255.255.255.0 we had THIS network:
192.168.10.0 >>> This is the "Network-IP" which is NOT usable for Host-PCs
192.168.10.1
192.168.10.2
192.168.10.3
192.168.10.4
192.168.10.5
...
...
...
192.168.10.253
192.168.10.254
192.168.10.255 >>>This is the Broadcast-IP, which is NOT useable for Host-PCs
Now with Subnetmask 255.255.255.128 we have THIS two networks:
First Subnet:
192.168.10.0 >>> This is the "Network-IP" which is NOT useable for Host-PCs
192.168.10.1
192.168.10.2
192.168.10.3
192.168.10.4
192.168.10.5
...
...
...
192.168.10.125
192.168.10.126
192.168.10.127 >>>This is the Broadcast-IP, which is NOT useable for Host-PCs
Second Subnet:
192.168.10.128 >>> This is the "Network-IP" which is NOT useable for Host-PCs
192.168.10.129
192.168.10.130
192.168.10.131
192.168.10..132
192.168.10.133
...
...
...
192.168.10.253
192.168.10.254
192.168.10.255 >>>This is the Broadcast-IP, which is NOT useable for Host-PCs
The Subnetmask defines how big the subnet is.
That means - how many Client-PCs will have place in that subnetwork.
A Subnetmask of 255.255.255.0 means in binary
11111111.11111111.11111111.00000000
So, what do we see?
4 Blocks, divided with a ".". Each of these blocks is also called "octett". Because - each Block has 8 bits.
To be able to do subnet-calculation, we first must understand binary calculation.
Lets take the first block.
The first "1" stands for a 128.
The second "1" stands for a 64.
The third "1" stands for a 32.
The fourth "1" stands for a 16.
The fifth "1" stands for a 8.
And so on. That means:
11111111=255
11110000=240
11100000=224
If we see something like "/24", that means that 24 bits are set to "1", from the left side.
Examples:
/16 = 255.255.0.0 = 11111111.11111111.00000000.00000000
/20 = 255.255.240.0 = 11111111.11111111.11110000
If we would take a subnetmask of 255.255.255.255 that would be
128+64+32+16+8+4+2+1.128+64+32+16+8+4+2+1.128+64+3 2+16+8+4+2+1.
128+64+32+16+8+4+2+1
and in binary it would be
11111111.11111111.11111111.11111111
Calculation of Subnet mask big enough for a specified number of Hosts
If they ask..
"create a subnet with minimum 10 host IPs"
than
1. calculate a power of two, that is minimum 10
2^3=8. That is not enough
2^4=16 That is higher than 10. Good.
2. Now put the LAST 4 Bits of your subnetmask to 0.
11111111.11111111.11111111.11110000
That is in decimal
255.255.255.240
With THIS Subnetmask, you have minimum 10 Host-Ips in the Subnet, without wasting to much IP-Addresses.
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