CCNA – How to Subnet

CCNA Aspirants, We are starting a set of tutorials for you guys to learn the concepts of CISCO Certification the EASY Way

Today we start with SUBNETTING, one of the easiest if you get the Subject matter, can ruining your CCNA  preparation  if you dont.

What is subnetting?
“Subnetting is basically breaking down a group of IP addresses in to smaller groups a way of managing IP addresses. With IPv4 we have a limited amount of IP adds that we can use so we have to manage them by subnetting.”

Let us consider this IP address
192.168.10.0
IP address is made up of 4 octects.
Eg.
192.168.10.1 192.168.10.2…. 192.168.10.250 and so on
Why there is no 192.168.10.256 or 192.168.10.300 or 300.1.5.258 ?

Here is why

IP addresses are represented in bit wise pattern
IP address is 32 bits (4 octets)
00000000.00000000.00000000.00000000
0. 0. 0. 0

11111111.1111111.11111111.1111111
255.255.255.255
So 0 is the minimum value  and 255 is the maximum value.
For every network there should have to be a network ID and Broadcast ID. The first available IP address will be the network ID and the last one will be the broadcast ID

In this case.
192.168.10.0 :- The first available IP address
192.168.10.255 :- The last available IP address

First one will be the network ID and the last one the broadcast ID.
This rule is valid for every networks.

Consider a case.

You are in a situation where you have only one address pool or range and you are to create two networks from that pool. That is where the real subnetting test begins…

Consider 192.168.10.0 is that range.

In normal case
If we give an IP from the network 192.168.10.0,  the default subnetmask will be 255.255.255.0
192.168.10.0
255.255.255.0 /24

No. of hosts = ?
For Class C networks, consider only the last octet of the subnetmask.

Last octet 0
Binary of 0 = 00000000
Then from that find the no. of networks and hosts

2 raised to no. of ones in that octet will be the no. of networks
2 raised to no. of zeros -2 will be the no. of hosts (explaination follows)

00000000
no.of ones :- 0 0*1= 1
no. of zeros :- 8 2*8-2 = 256-2 = 254

So the network range
192.168.10.0
192.168.10.1

192.168.10.253
192.168.10.254
10.192.168.10.255

So as per our rules

192.168.10.0 – Network ID
192.168.10.1 – First available host
192.168.10.254 – Last available host
192.168.10.255 – Broadcast ID

NOW let the SUBNETTING begin
As we discussed previously we need two networks within a Class C range that is 192.168.10.0
We all know if we use different Class C networks it is possible to have two networks.
But we don’t want to waste our IP addresses, So what we will do is we are going to create two networks in that range

That is from 192.168.10.0 subnet, we will create 2 networks

192.168.10.0 255.255.255.0 is the mask that we used
For class C the first three parts are network part and the last one is the host part.
We only have the option to change the last part. .ie, the host part

We can’t change the value like what we decide,  we can only change it in bit wise

255. 255. 255. 0
11111111.1111111.11111111.00000000
we are changing the 0 bit to one (last octet)

11111111.1111111.11111111.10000000
255. 255. 255. 128

No. of networks = 2*1 = 2
No. of hosts = 2*7-2 = 128-2 = 126
Block size = 256-128=128

The subnet range
Network 1
192.168.10.0 :- Network ID
192.168.10.1 :- First host
192.168.10.126 :- Last host
192.168.10.127 :- Broadcast ID

Network 2
192.168.10.128 :- Network ID
192.168.10.129 :- First host
192.168.10.254 :- Last host
192.168.10.255 :- Broadcast ID

That’s about it folks, you can become a great  SUB-NETTERS  :)
If you want some examples, please comment.
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Dhawal D
  • Dbshah30

    Can you send link to more examples of Class A and B networks.. I am studying this subject now and have to use Subnetting every now and then.. Thanks.