Now the tricky part, find a ‘magic number’ which is 256 minus mask’s interesting octet.For any octets fully to the right of the interesting column, put 0s in the subnet number.
For any octets fully to the left of the interesting column, copy value(s) from the original IP Address into all addresses(subnet number, first-last address and broadcast address). I will define the column that contain the difficult number(not 255s and 0s) as the interesting column.Let see the way to figure out in few seconds. In this case, most people find it’s difficult to calculate and some has to do by binary math which is time consuming.
To find the last valid IP address, copy the broadcast address and subtract 1 to the fourth octet.To find the first valid IP address, copy the subnet number and add 1 to the fourth octet.
This is the same in ‘Find the subnet number’, step 1 I guess many people know this already, but l will explain a little bit to revise for someone. I divide into 2 categories which depend on subnet mask:įor the masks that contain only 255s and 0s.