Cisco CCNA: Subnetting class C address made easy, from start to finish.

Statistics have shown
that apart from access control list and OSPF, the major area that CCNA students
encounter challenges while studying for their certification is subnetting.
While there are many applications to you help subnet, knowing how to get by
manually is very important, after all, you must do it manually to pass the
exam. So in this article, I will be sharing how to subnet the class C address
with a detailed example.

Subnetting is simply the process of taking a block of IP
and breaking it down into smaller blocks called subnets. See it like set and
sub-sets in mathematics.  Subnetting helps segment a network, bringing
about network security while helping engineers to avoid broadcast storm.

Subnetting a class C
Range of a class C
 if the
values in the first octet of an IP address fall between 192 and 223, then it is
a class C address. Example, 192.x.x.x to 223.x.x.x.
Now, assuming we have, with this network address and subnetmask, no subnetting is done
because /24 which equals, is the default subnetmask for class C
addresses. Subnetting starts when you have subnetmasks from /25 to /30. /31 and
/32 cannot be subnetted because they will leave you with no IP addresses for
Example:lets subnet
Step one: write out the subnet mask from
the given slash notation. /27= (/27 means 27 bits on. Each 8 bit
is 255 while the last octet of 3 bits equals 224 in decimal)
Step two: determine the number subnets
that the subnetmask above will give. Here you use this formula: 2^X, where x is equal to the number of
bits borrowed. Since the IP given is a class C IP and the default subnetmask
for Class is /24 but we are given /27 in this example, it therefore means we
have borrowed 3 bits. Substituting 3 into the formula, we have 2^3=8. This
means we well have 8 subnets.
Step 3: How many hosts will the given
network address and subnetmask produce per subnet? To answer this question, you
will use this formula; 2^y-2, where y equals the number of bits off. If in the
question we were given /27 out of a total of 32 (IP version 4 is a 32-bit
address), it means we have 32-27=5. We have 5 bits off. This gives us 2^5-2. This
will be 32-2 which gives us 30. (The 2 we subtracted is for the broadcast and
network addresses. You cannot assign those addresses to hosts on your network).
From here we know that we will have 30 valid hosts for each of the 8 subnets we
will get from Next is to determine the range of these subenets
as well as the valid IPs in each subnet.
Step 4: Block size. We will use block
size to get the range of the subnets as well as the valid IPs in each subnet.
To get the block size you subtract the first non-255 value in the given
subnetmask from 256. 256 is a constant value and in this example, 224 is the
first non-255 value in our subnetmask; Block size=256-224=32. With a block size
of 32, our subnets will be:

You may also like: subnetting class B, from start to finish.

First IP
Last IP

You can see a video on this here.

 That is all you need to answer any question regarding class C subnetting. A post on class B subnetting will be coming soon.

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Ashioma Michael, a BSc (Computer Science)., MTCNA, CCNA, and CCNP holder with many years of industry-proven experience in network design, implementation and optimization. He has tutored and guided many professionals towards obtaining their Cisco certifications. Mike works as a senior network engineer with one of the leading internet service providers in West Africa.

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