#### My CCNA experience

- Posted on 2015-07-21 04:07:08
- 644 Views
- 3 Uptweets
- 0 Favorites
- 0 Likes
- 0 Dislikes

Welcome,

**ATTACKING A
SUBNET **

Many students nowadays have questions and difficulty understanding
what subnetting is. I have made an easy explanation on how to solve subnetting
questions to ace the test and to use it in the real world. I will be honest,
more than likely you will not subnet a network by memory when you get hired for
a networking job. The entire network is already designed. Calculators and
programs have been made to ease the pain. But, the purpose of this tactic is
for you to attack it on the day you won’t have a calculator in hand, like your
test day. Hey maybe you forgot how to subnet because you have been so used to
using a calculator like me. Before you continue, you must have some
understanding of what subnet masks are and their purposes. If you don’t, then
you must go back and hit the books.

First thing is first, having a visual chart will help you
tremendously when subnetting a network. So let’s first start with this chart.
When the day comes that you have to take your CCNA test, I would HIGHLY
recommend writing out this simple chart quickly before the test starts. The
test does not have enough time for you to spare.

**128** 64 32 16 8 4 2 1

192

224

240

248

252

254

255

So what do you do when you have 192.168.1.0 /25 mask? Well
you must know that a subnet mask is composed by 8 bits for every decimal.

255.255.255.255 = 8bits.8bits.8bits.8bits = 32bits

So what happens when you have a /25 mask? You just add the
first 3 decimals equal to 24 bits. Then you only have 1 left right?

255.255.255.? = 255.255.255. (Add the first one from the
graph horizontally which is 128)

255.255.255.128 is your subnet mask

What about a /26 mask? 255.255.255.? = 255.255.255. (Add 2
from the graph horizontally which is 128 + 64 = 192)

**128 64** 32 16 8 4 2
1

**192**

224

240

248

252

254

255

One more example, what about a /27 mask? = 255.255.255. (Add
3 from the graph horizontally 128+64+32= 224)

**128 64 32** 16
8 4 2 1

**192**

**224**

240

248

252

254

255

As you can see, your answers are already in the graph. You
just need to add the same amount of bits that you used horizontally to
vertically. Now, what do you do when you have the subnet mask? You subtract 256
from whatever you have left on the subnet mask. Using a 192.168.1.0 /25 mask,
you subtract 256 from 128 and that leaves you with a 128 subnet block. This
means that you only have 2 subnets to use. Remember to subtract 2 for the
subnet ip address and broadcast ip address. EASY RIGHT?!!!!

**256-128 = 128 subnet block**

**192.168.1.0 ****ß**

**192.168.1.128 **

**192.168.1.256**

** **

**SUBNET IP MASK**

192.168.1.0 255.255.255.128

**USABLE IP
ADDRESSES
BROADCAST IP **

192.168.1.1 to
192.168.1.126
192.168.1.127

After attacking this /25 subnet, now you know that you can
have 2 subnets and 126 hosts in your network.

Ok one more that is not a common one. What is the subnet
mask and usable IP addresses for 15.25.13.16 /28? =

We already know the first 3 decimals are 24 bits. Add 4 bits
to make it 28.

128 64 32 16 8 4 2 1

192

224

240

248

252

254

255

**256-240= 16 subnet block**

**15.25.13.0**

**15.25.13.16 ****ß**

**15.25.13.32**

** **

**Subnet IP** 15.25.13.16 **Mask** 255.255.255.240

**Usable IP
Broadcast IP **

15.25.13.17 to 15.25.13.30 15.25.13.31