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CCNA™: Subnet masking -II


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What we discussed in the previous section is Classful subnet masking. A Subnetmask normally contains the host portion of the bits also. This is called Classless Inter Domain Routing (CIDR). This will enable more networks for a given class of network address. For example, allowing 3 host bits towards subnet portion in our previous IP address, will allow us to offer 2X2X2 or 8 additional subnetworks. Traditionally, all zeros, and all ones subnets are not used, and hence we are left with 6 subnets. 1100 0000.1011 1101.1101 0010.0100 1110 Class C IP Address 1111 1111.1111 1111.1111 1111.1110 0000 Class C subnet mask with 3 additional bits of host portion used for Subnetting. 

Broadcast address: 1100 0000.1011 1101.1101 0010.0101 1111 :

The above is the broadcast address for a given subnet ( Under Classful routing, the broadcast address would have been

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 2or 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:, 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 ISP's customers such as individual or Corporates are then allocated networks from the big ISP's 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 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


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.

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