Subnetting Case

Subnetting (Sub Networking) is the act of dividing a larger network into smaller networks. In this document I'm going to explain subnetting in the way that it finally made sense to me. I'm not going to get into binary because all it did was confuse me in the beginning. You will be able to apply binary easier once this section is out of the way.

Subnets are only going to be in the following sets. These sets can be applied to all network octets except the first:

0 = /24

128 = /25

192 = /26

224 = /27

240 = /28

248 = /29

252 = /30

128 translates to /25. The /25 is how many bits are being used. In an IP Address there are 32 slots(bits). The /25 means the first 25 of the 32 slots is being used. In binary this would appear as twenty-five 1's and seven 0's.

128 also translates to 255.255.255.128 in a class C network. In a class B network, the translation would be 255.255.128.0.

When I was able to memorize these 6 numbers (128, 192, etc), subnetting became very easy.

Start with 256. 256 is the total number of addresses available in an octet of an IP address. 256 is not a valid address; the numbering range is 0-255 which is 256 numbers.(0 counts as #1)

Back to the set of subnets:

128 divides the 256 by 2. This will take the 256 network and divide it into 2 networks: 0-127, and 128-255.

192 divides the 256 by 4. This will take the 256 network and divide it into 4 networks: 0-63, 64-127, 128-191, 192-255.

224 divides the 256 by 8.

240 divides the 256 by 16.

248 divides the 256 by 32.

252 divides the 256 by 64.

The easiest way to find the network increments is to use the Magic Number System. Remember the subnet sets?

Here we go:

128:

Take 256 and subtract 128. 256-128=128. 128 is your magic number. There are 128 host addresses in this subnet. A new network will start after every 128 bits. (0 through =128 bits) (128 through 255 =128 bits). 2 Networks

192:

Take 256 and subtract 192. 256-192=64. 64 is your magic number. There are 64 host addresses in this subnet.

A new network will start after every 64 bits. (0 through 63 =64 bits, 64 through 127 =64 bits, 128 through 191 =64 bits, 192 through 255 =64 bits). 4 networks.

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Now what?

When you are given a subnet problem it will contain an IP address with a subnet. It will then either ask you to create subnets from a required host address number, or ask you to find the network range the IP belongs to.

First Example:

Find the subnet that 192.168.1.71/27 belongs to.

1. Remember the dotted decimal equivalent:

/27=224

2. Calculate the Magic Number:

256-224=

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