Classless Inter Domain Routing (CIDR)

 CIDR is  a new classification defining the network range to avoid IPV4 Congestion. Before Understanding CIDR let us look into the below pic.

  

Lets look at the above diagram , it shows the number of available ips in the network , so we will have upto 2^32. Let us understand the circle is a large cake and it is being cut into slices of  class A/B/C/D/E/F.

So when we approach IANA(Internet assigned number Authority ) ,we have to buy or take entire slice of IPS irrespective of your need.

For example class A is having 2^24 and class B is having 2^16 (16000).

class A : 8 N/24 HID

class B: 16 N/16 HID

Class C: 24N/8HID

To solve this problem we have to go with classless representation which avoids taking the IP range from the entire class. Here you will be asked to take a small slice of cake upon your requirement in the multiples of 2^n and takes the below representation

CIDR NOTATION:

a.b.c.d/n = BID 32-n and n means host id 

eg : 10.20.1.20/20 

BID= 20 and host id =12 

rules of CIDR:

1.  All IPS should be continuous
2. Range should be size of 2^n  (n-k|k)  n bit binary number divided by 2 ^k the reminder will be K
3.  First Ip address should be evenly divisible by size of the block

Example : 10.1.2.32---------10.1.2.47 => 16 ips