Subnetting Tutorial – Subnetting Made Easy
In this article, we will learn how to subnet and make subnetting an easy task.
The table below summarizes the possible network numbers, the total number of each type, and the number of hosts in each Class A, B, and C network.
| Default subnet mask | Range | |
| Class A | 255.0.0.0 (/8) | 1.0.0.0 – 126.255.255.255 |
| Class B | 255.255.0.0 (/16) | 128.0.0.0 – 191.255.255.255 |
| Class C | 255.255.255.0 (/24) | 192.0.0.0 – 223.255.255.255 |
Table 1 – Default subnet mask & range of each class
Class A addresses begin with a 0 bit. Therefore, all addresses from 1.0.0.0 to 126.255.255.255 belong to class A (1=0000 0001; 126 = 0111 1110).
The 0.0.0.0 address is reserved for default routing and the 127.0.0.0 address is reserved for loopback testing so they don’t belong to any class.
Class B addresses begin with a 1 bit and a 0 bit. Therefore, all addresses from 128.0.0.0 to 191.255.255.255 belong to class B (128=1000 0000; 191 = 1011 1111).
Class C addresses begin with two 1 bits and a 0 bit. Class C addresses range from 192.0.0.0 to 223.255.255.255 (192 = 1100 0000; 223 = 1101 1111).
Class D & E are used for Multicast and Research purposes and we are not allowed to subnet them so they are not mentioned here.
Note: The number behind the slash notation (/) specifies how many bits are turned on (bit 1). For example:
+ “/8″ equals “1111 1111.0000 0000.0000 0000.0000 0000″ -> 8 bits are turned on (bit 1)
+ “/12″ equals “1111 1111.1111 0000.0000 0000.0000 0000″ -> 12 bits are turned on (bit 1)
+ “/28″ equals “1111 1111.1111 1111.1111 1111.1111 0000″ -> 28 bits are turned on (bit 1)
+ “/32″ equals “1111 1111.1111 1111.1111 1111.1111 1111″ -> 32 bits are turned on (bit 1) and this is also the maximum value because all bits are turned on.
The slash notation (following with a number) is equivalent to a subnet mask. If you know the slash notation you can figure out the subnet mask and vice versa. For example, “/8″ is equivalent to “255.0.0.0″; “/12″ is equivalent to “255.240.0.0″; “/28″ is equivalent to “255.255.255.240″; “/32″ is equivalent to “255.255.255.255″.
The Network & Host parts of each class by default
From the “default subnet mask” shown above, we can identify the network and host part of each class. Notice that in the subnet mask, bit 1 represents for Network part while bit 0 presents for Host part (255 equals to 1111 1111 and 0 equals to 0000 0000 in binary form).
What is “subnetting”?
When changing a number in the Network part of an IP address we will be in a different network from the previous address. For example, the IP address 11.0.0.1 belongs to class A and has a default subnet mask of 255.0.0.0; if we change the number in the first octet (a block of 8 bits, the first octet is the leftmost 8 bits) we will create a different network. For example, 12.0.0.1 is in a different network from 11.0.0.1. But if we change a number in the Host part, we are still in the same Network. For example, 11.1.0.1 is in the same network of 11.0.0.1.
The problem here is if we want to create 300 networks how can we do that? In the above example, we can only create different networks when changing the first octet so we can create a maximum of 255 networks because the first octet can only range from 1 to 255 (in fact it is much smaller because class A only range from 1 to 126). Now we have to use a technique called “subnetting” to achieve our purpose.
“Subnetting” means we borrow some bits from the Host part to add to the Network part. This allows us to have more networks than using the default subnet mask. For example, we can borrow some bits in the next octet to make the address 11.1.0.1 belong to a different network from 11.0.0.1.
How to subnet?
Do you remember that I said “in the subnet mask, bit 1 represents for Network part while bit 0 presents for Host part”? Well, this also means that we can specify how many bits we want to borrow by changing how many bit 0 to bit 1 in the subnet mask.
Let’s come back to our example with the IP 11.0.0.1, we will write all numbers in binary form to reveal what a computer really sees in an IP address.
Now you can clearly see that the subnet mask will decide which is the Network part, which is the Host part. By borrowing 8 bits, our subnet mask will be like this:
After changing the second octet of the subnet mask from all “0″ to all “1″, the Network part is now extended. Now we can create new networks by changing number in the first or second octet. This greatly increases the number of networks we can create. With this new subnet mask, IP 11.1.0.1 is in different network from IP 11.0.0.1 because “1″ in the second octet now belongs to the Network part.
So, in conclusion we “subnet” by borrowing bit “0″ in the Host portion and converting them to bit “1″. The number of borrowed bits is depended on how many networks we need.
Note: A rule of borrowing bits is we can only borrow bit 0 from the left to the right without skipping any bit 0. For example, you can borrow like this: “1111 1111. 1100 0000.0000 0000.0000 0000″ but not this: “1111 1111. 1010 0000.0000 0000.0000 0000″. In general, just make sure all your bit “1″s are successive on the left and all your bit “0″s are successive on the right.
In the next part we will learn how to calculate the number of sub-networks and hosts-per-subnet
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In example 3, /28 will give us 2^4=16 subnets.Isn’t it so???
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There is a mistake in exercise 3 as we borrowed 6 bits not 2 bits, so:
No. of subnets=64
Please rectify this error.
Thanks
Abdullah
Ok when I continue reading I realized that it was not a mistake as we depended on the assigned subnet mask not the default subnet mask.
Thanks for the accurate information.
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Okay, I get the math for finding subnets, right up until you did this…
The initial network 198.23.16.0/28 is also the first subnet, so:
+ The first subnet: 198.23.16.0/30
+ The second subnet: 198.23.16.4/30
How or why did you go from /28 to /30? As much as I read and work the math out, I’m not seeing how or why you made this transition. I think it has something to do with planned vs assigned subnet mask as stated near the bottom of page 2, but I’m just not getting it. Do you have any other ways to explain it?
Thanks!
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Okay, never mind. I was reading the question wrong. Doh!! :-)
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The first subnet: 198.23.16.0/30
+ The second subnet: 198.23.16.4/30 because the increment is 4 so we plus the network address with it to get the next network address (0 + 4 = 4)
+ The third subnet: 198.23.16.8/30 (4 + 4 = 8)
+ The fourth subnet: 198.23.16.12/30 (8 + 4 = 12)
I dint understand this Part
i cannot go further without it . please someone help
+ First subnet: 198.23.16.0/30 (the 4th octet is 00000000)
+ Second subnet: 198.23.16.4/30 (the 4th octet is 00000100)
+ Third subnet: 198.23.16.8/30 (the 4th octet is 00001000)
+ Fourth subnet: 198.23.16.12/30 (the 4th octet is 00001100)
I meant this part
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@Cynthia: This link explains “ip subnet-zero” very well, please read it: https://learningnetwork.cisco.com/thread/8388
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hello anybody.
i have the problem in the subneting so i’ve problem in the class a
who can help me?
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Example 3 is bit confusing
Why the broadcast address is 198.23.16.3?
I know the first address is always the router address but I thought the last address is the broadcast right? But 198.23.16.3 is not the last address.
omg I just realized that you gave the specific IP in the question 198.23.16.0/28. And with /30 and for this IP, the first address of the next subnet is 198.23.16.4 /30 and so the address right before that must be the broadcast address for the 1st subnet which is what we want.
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If you need help learning to subnet, etc. Here is a video that I used to refresh and/or learn from:
http://www.youtube.com/watch?v=rs39FWDhzDs
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