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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.
192.189.210.078: 1100 0000.1011 1101.1101 0010.0100 1110 Class C IP Address
255.255.255.224: 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 (192.189.210.078).
Under Classful routing, the broadcast address would have been 22.214.171.124.
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.
Normally, ISPs allocate the IP addresses for individuals or
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.
You have an IP of 126.96.36.199 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
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.