Mastering subnetting is a fundamental skill in network engineering. Finding the subnet ID given an IP address and a non-trivial subnet mask can be a challenge at first, but with practice, it can become a second nature making possible to solve subnetting problems in just a few seconds.

One of the main study resources I used for the CCNA was the Cisco Official Cert Guide Book (OCG). In this book, the author describe a process to find the subnet ID when using a difficult mask. A difficult mask is a subnet mask that has one octet that is neither 0 nor 255. Since, by definition, a subnet mask cannot interleave 0s and 1s, the possible values for that octet are: 128, 192, 224, 240, 248, 252 and 254. The OCG book calls that octet the interesting octet.

The process of finding the resident subnet ID of an IP address described in the book can be summarized as follows. For every octet in the subnet mask:

1. If the mask octet is 255, just copy the corresponding IP octet in the same position.
2. If the mask octet is 0, put a 0 in the same position.
3. If the mask octet is an interesting octet:
• Calculate the magic number as 256 – V, where V is the decimal value of the interesting octet.
• Calculate the subnet ID octet as the largest multiple of the magic number that is less than the correspoding IP octet.

As an example, let’s find the resident subnet ID of the IP address 10.0.50.76/29. In this case, the subnet mask in dotted decimal notation (DDN) is 255.255.255.248. Following the previous procedure, since the first three octets are 255, to find the resulting subnet ID we just need to copy the first three octets of the IP address: 10.0.50.X. To find the last octet ‘X’, we have to follow the step 3 and calculate the magic number as 256 – 248 = 8. The value ‘X’ will be the largest multiple of 8 that is less than 76, which is 72. Therefore, the subnet ID will be 10.0.50.72/29.

But, at this point, you may ask yourself: why does it work? In order to understand this process, let’s have a look at the following figure:

The figure shows four different subnetting schemes. The first one corresponds to a /25 mask, which divides the original network in two subnets. The second one corresponds to a /26, which divides the network in four subnets. The next one divides the network in eight subnets (/27) and the last one in sixteen subnets (/28). The figure highlights one important fact: in all subnetting schemes, the last subnet octet always corresponds to the interesting octet described in the OCG book. This is because the interesting octet, by definition, will have a binary value of all 0’s in the host portion of the octet and all 1’s in the subnet portion. Another important fact we can notice from the figure is that the size of the resulting subnets can be calculated by subtracting the interesting octet value from 256, which corresponds to the magic number concept used in the OCG book.

Once we know the size (magic number) of the resulting divisions of the original network range, finding the multiple of this number closest to the original IP octet value without going over gives us the desired resident subnet ID.