How to Calculate Subnetting
Subnetting is a method of breaking apart the available IP addresses on your network into smaller pieces. Subnetting creates better network implementation because users with similar functions can be grouped in the same subnet. Routers do not have to work as hard because they only need to route information going from one subnet to the next needs to be routed instead of every single bit of information passing between every user. This saves time and resources while improving network stability.
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Instructions
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Determine what class your IP address is in. If your IP address begins between 0 and 127, then it is a Class A address. If your IP address begins between 128 and 191, then it is a Class B address. If your IP address begins between 192 and 223, then it is a Class C address.
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Determine how many subnet bits are in each octet of your subnet mask. A subnet mask is comprised of four numbers (known as octets) separated by periods. Use the following to determine how many subnet bits are in each octet: 0 = 0 subnet bits, 128 = 1 subnet bit, 192 = 2 subnet bits, 224 = 3 subnet bits, 240 = 4 subnet bits, 248 = 5 subnet bits, 252 = 6 subnet bits, 254 = 7 subnet bits, and 255 = 8 subnet bits. Disregard the first octet for Class A IP addresses, the first and second octets for Class B IP addresses, and the first 3 octets for Class C addresses. Thus, if your IP address is 192.168.10.0, and your subnet mask reads 255.255.255.192, this means you have 2 subnet bits.
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Raise 2 to the power of the number of subnet bits. For instance, if you found your subnet mask to have 8 subnet bits, you would raise 2 to the 8th power. This is the total number of subnets your network has.
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Subtract the last octet in your subnet mask that does not have "0" from 256. For instance, if you have a subnet mask 255.255.240.0, you would subtract 240 from 256 to come up with 16. This is the size or "block" of each subnet.
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Use your IP address to determine the lowest IP address for your network. To determine the lowest possible IP address: If you have a Class A address, keep the first octet and make the other octets 0 (for instance, 10.0.0.0). If you have a Class B address, keep the first two octets and make the other octets 0 (for instance, 135.20.0.0). If you have a Class C address, keep the first three octets and make the last octet 0 (for instance, 192.168.10.0).
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Add the block size of the subnet to the lowest IP address until you reach the first octet for Class A address, second octet for Class B addresses or third octet for Class C addresses. If you have a Class A or B subnet, once you exceed 255 on the last octet, increase the third octet by one, but keep increasing the last octet by the size/block of the subnet. In only Class A addresses, once you exceed 255 on the third octet, increase the second octet by one, but keep increasing the last octet by the size/block of the subnet. For example, if you have a Class B address of 130.20.0.0 and your subnet mask is 255.255.255.192, then your subnets would be 130.20.0.0, 130.20.0.128, 130.20.1.0, 130.20.1.128, and so on until you reach 130.20.255.128.
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