Subnetting – The Magic Number

Subnetting – The Magic Number

If you want to know about computer networking, then you should complete the CCNA / CCAI course. In this course, you will find the Subnetting chapter and this chapter is most important for you guys, believe me. If you fail to understand Subnetting chapter than you will also fail to understand computer networking.I’ll help you to learn by using Subnetting – The Magic Number pdf.

computer networking

I’ll teach you subnetting by using the “magic number” method.

Believe me, it’ll very helpful for you.

Who am I?

I’m MD. Soliman Al Helal. I’m a Digital Marketing Expert. and co-founder of mileniumit.com.

I completed the CCNA course on July 10, 2017.

However, here is my CCNA certificate.

My CCNA certificate

• If you have a basic idea about computer networking and you are also good in English.
• Or, you are a running student of CCNA/CCAI.

This EBOOK is 100% unique. You’ll not find it anywhere.

This EBOOK is 100% unique

I’m giving in this article a 100% solution of subnetting.

Warning:

I’m not providing here any video or audio tutorial.

It’s just an Ebook.

However, let’s start the class.

The first part of Subnetting – The Magic Number

The easiest way to subnet an address is by using the “magic number” method. Here’s how it works:

So, in order to subnet, we need to know a couple of things:

1. The network address we’ll be subnetting
2. How many subnets or hosts we need

Remember the rules of subnetting:

1. You must borrow at least 2 bits
2. You must leave at least 2 bits

We must also be able to identify classes of addresses by looking at the first octet:

1 – 127 –  Class A  (127 reserved for loopback testing)

128 – 191 – Class B

192 – 223 – Class C   (192 used for local intranets)

Depending on the class, a portion of the address (Network) is assigned to us, the remaining portion is the Host portion we can use to subnet.

Class A = N . H . H . H

Class B = N . N . H . H

And, other is

Class C = N . N . N . H

The Host portion of an address (the H) is the octet(s) we can borrow from for subnetting.

What does this mean?

Take a Class C address:

200.100.11.0 (200 means Class C)

The number in binary: 11001000  .  01100100  .  00001011  .  00000000

Since this is a Class C, we’ll be borrowing from the LAST octet. (00000000)

How many subnets do we want?

Remember: You must borrow at least 2 bits, leave at least 2 bits.

Range  = 11000000 to 11111100  (borrowing at least 2, leaving at least 2)

You can determine your subnet(s) either by deciding how many subnets

you want or how many hosts per subnet you want.

Either way, the number of bits borrowed will be a power of 2:

In class C, if we borrow 4 bits (11110000), that’s 4 bits of subnetting.

Number of subnets is 2 x 2 x 2 x 2 (2-to-the-4^th) or 16 subnets.

The number of hosts per subnet is determined by the bits not borrowed (the 0’s.  In this case we have 4 zeros (2 x 2 x 2 x 2) or 16 hosts/subnet.

**Binary Note**

To make life a little easier for you, you should be accustomed to the patterns of bits in a mask or any byte) so you can immediately determine a mask just by knowing how many bits are borrowed. In other words, you can do it “in your head” by remembering the following:

*Note that you cannot borrow only 1 or 7 or 8 in a Class C, but remember, if you have an A or B, your borrowing may span 2 or more octets, so in a Class B borrowing 9 bits (xxxxxxxx.xxxxxxxx.11111111.10000000), you should immediately know the mask by looking at the octets. (3^rd is 255, 4^th is 128)

O.K. Back to the original problem:

• 200.100.11.0 and we want to borrow 4 bits:

11111111.11111111.11111111.11110000 is our mask.

(Remember: The subnet mask is when all the network and subnet bits, if any, are set to 1)

By looking at the binary representation, we know the mask in decimal is:

255.255.255.240

What we must do now is determine what our network numbers are for each subnet.  We borrowed 4 bits which gives us a mask of 240 (last octet):

What is our “Magic Number”?  The Magic Number is the increment of our subnet addresses.  We obtain the magic number by subtracting the LAST NON-ZERO OCTET in our subnet mask from 256 (the number of possible numbers represented in 1 byte: 0-255 inclusive):

…so,    256 – 240 = 16, our magic number.

Now, let’s write out our network numbers, useable hosts and broadcasts:

^^^^

Note:  You add the magic number (16) to each network number to get the next subnet address.

Remember, you have 2 subnets you cannot use: the very first and the very last. The first is a network subnet, the last is a broadcast subnet. We cannot use any IP’s on either of those subnets.  The same rule applies to the first and last addresses on each subnet.

What about a Class B?

Class B address are done the same, but remember you have the last 2 octets to work with:

136.122.0.0, unsubnetted mask: 255.255.0.0

Let’s write out the mask in binary:

see this    11111111 . 11111111 . 00000000 . 00000000

If I must borrow at least 2 host bits and leave at least 2 host bits, I can borrow from 2 to 14 bits (using the last 2 octets).  Let’s borrow 9:

11111111 . 11111111 . 11111111 . 1000000

Our mask is:  255.255.255.128

This gives us:

2x2x2x2x2x2x2x2x2 (9 borrowed) or 512 subnets (510 useable)

2x2x2x2x2x2x2 (7 left) or 128 hosts / subnet (126/subnet useable)

Magic number:  256 – 128 (last non-zero octet) = 128

Original address:  136.122.0.0   Magic #128

So, is the first useable address 136.122.0.128 or 136.122.128.0?

Since the last non-zero octet was the last octet, that’s where we start our network increments:

•     136.122.0.0 (cannot use)
  •    136.122.0.128  (added 128)

136.122.0.256…….hang on!! Can’t do this!!!

Here’s 136.122.0.0:

10001000.01111010.00000000.00000000

256 is NOT a valid decimal number in 1 byte  (range: 0 –255)

10001000.01111010.00000000.11111111   (That’s 255!)

Now what?

We roll over the next bit in the previous octet:

10001000.01111010.00000001.00000000

• 136.122.1.0– the next subnet..

136.122.1.128 (add 128)

136.122.2.0 (add 128)

136.122.2.128     … and so on.

We are actually carrying the extra 1 from 256 (since we can only represent 255 in an octet) in the last octet, which increments the previous octet.

Subnetting – The Magic Number

Hope this helps!!

Learn more…..

Basic Router and Switch Configuration