Finding the Subnet ID: Difficult Masks
The following written process lists all the steps for find the subnet ID, using only decimal math. This process adds to the earlier process used with easy masks. For each octet:
Step 1. If the mask octet = 255, copy the decimal IP address.
Step 2. If the mask octet = 0, write a decimal 0.
Step 3. If the mask is neither, refer to this octet as the interesting octet:
A. Calculate the magic number as 256 – mask.
B. Set the subnet ID’s value to the multiple of the magic number that is closest to the IP address without going over.
The process uses two new terms created for this book: magic number and interesting ...
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