Subnetting in 11 steps

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Subnetting in 11 steps

As seen on, here is my ip addressing article in full...

Original Subnetting in 11 Steps article

Subnetting in 11 Steps

There are a few things that you will need to know first. I personally use 11 rules that I learned from Mike Vana. Below you will find the 11 rules as well as some examples.

The best thing to explain the different rules would be to give a word problem that requires utilizing the rules.

For this example, we are give the address of with a subnet mask of You need to break this default network up into at least 6 networks. Below we will go through each step of the eleven rules.

1. What Class are you dealing with?

Before you can begin subnetting you must know what class of address you are dealing with.
* Class A: 0-126
* Class B: 128-191
* Class C: 192-223
So to determine the class of a specific IP address you will need to look at the first octet i.e. would be a Class A address

2. What is the default subnet mask for that class of address?

Rule 2: What is the Default subnet mask for the Class of address you are dealing with?
Each Class of IP address has a Default Subnet mask:
* Class A:
* Class B:
* Class C:
Continuing with the example for Rule 1, We are dealing with a Class A IP address so our Default Subnet Mask is:

3. What is the specific subnet you need to use?

First we need to look at a little table will be added to as we go

If we look at the initial problem, you will see that we need at least 6 valid networks. According to the table, in order to acheive atleast 6 valid networks we need to borrow 3 bits. This will give us a specific subnet mask of

4. How many valid subnets does rule 3 provide?

In order to figure out the number of networks the specific subnet mask will allow for you will need to convert the subnet mask to binary. 11111111.11100000.00000000.00000000
Since this is a class A address we can ignore the first octet because it is all 1s. but if we look from the second octet over to the right:> 11100000.00000000.00000000 we can figure out how many networks are available
You will count the 1s, in this case there are 3 bits borrowed. This gives us the equation 2 to the 3rd power minus 2

5. How many valid hosts does rule 3 provide?

Looking at the information we found in the last rule: > 11100000.00000000.00000000
We simply count the 0s and this gives us 2 to the 21st power minus 2: 2_21 - 2

6. What is your range?

Range is used to identify the actual network addresses for use in later rules. To find the specific range you will take the octet of the subnet mask that has been subnetted. In our case it is we will use 224 and subtract it from 256 to gain our range.

Our range is 32

7. What are all of the networks within that range?

To find all of the networks created we take our range and count by it just as you can count by two i.e. 0, 2, 4, 6, 8 and so on. In this case we are going to count by 32.

0, 32, 64, 96, 128, 160, 192, 224

We must start at 0 and continue until we reach the value of our subnetted octet 224 in this case.

8. What are the valid networks within that range?

To find that Valid networks we simply discount the first and last network created in the last rule.

32, 64, 96, 128, 160, 192

As you can see this gives us the same number of networks as in rule #4

9. What is your first valid host address in the first subnet?

The first valid host address in this case is: We cannot start with because that is the network ip address, in other words that is how the network as a whole is identified.

10. What is your last valid host address in the first subnet?

The last valid host address in the first subnet is

11. What is your broadcast address in the first subnet?

The broadcast address for the first network is as it is the last address in the range before the next network begins

Last edited by jking2100; 04-30-2007 at 11:09 AM.. Reason: mistake in hyperlink
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