CCNA: Subnet masking -II

Note that by using Subnetting, we are able to increase the number of networks available within a given IP address. On the otherhand, we will be loosing the number of hosts available within a subnet to 2⁴  or 16 hosts per subnet. Again, all zeros, and all ones host addresses are traditionally reserved for other purposes.
CIDR (Classless InterDomain Routing) notation: Subnet mask is also represented as below:
192.189.210.078/27, where 27 is the number of bits in the network portion of the IP address.
Why use CIDR?
Normally, ISPs allocate the IP addresses for individuals or Corporates.  The reason being that it is almost impossible to allocate a classful IP address to every individual or a corporate. Using CIDR, the biggest ISPs are given large pool of IP address space. The ISPs customers such as individual or Corporates are then allocated networks from the big ISPs pool. This kind of arrangement will enable efficient management and utilization of the Internet.
Classful addresses can easily be written in CIDR notation
Class A =  A.B.C.D/8, Class B = A.B.C.D/16, and Class C = A.B.C.D/24
Where A,B,C,D are dotted decimal octets.
Example Question:
You have an IP of 156.233.42.56 with a subnet mask of 7 bits. How many hosts and subnets are possible?

A. 126 hosts and 510 subnets
B. 128 subnets and 512 hosts
C. 510 hosts and 126 subnets
D. 512 subnets and 128 hosts

Correct answer: C
Explanation:
Class B network has the form N.N.H.H, the default subnet mask is 16 bits long.
There is additional 7 bits to the default subnet mask. The total number of bits in subnet are 16+7 = 23.
This leaves us with 32-23 =9 bits for assigning to hosts.
7 bits of subnet mask corresponds to (2^7-2)=128-2 = 126 subnets.
9 bits belonging to host addresses correspond to (2^9-2)=512-2 = 510 hosts.