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I have a (hypothetical) network with the following subnet addresses allocated using VLSM:

Allocated addresses

The network address is 150.200.48.0.

According to this site: http://subnet-calculator.org/supernets.php, you can calculate the required supernet mask by taking the length of the sequence of matching bits from left to right:

150.200.048.000: 10010110.11001000.00110000.00000000
...
150.200.052.092: 10010110.11001000.00110100.01011100

Longest matched: xxxxxxxx.xxxxxxxx.xxxxx---.--------

21 bits are matched, so the supernet mask will be /21.

Is this really the correct way of working this out? I assume that this calculation is correct as the supernet mask of /21 can be confirmed on various subnet mask calculators online.

For some reason I'm really struggling to find a good source for this information.

あらまあ
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  • Please have a look at this answer for example: [Calculating the Netmask Length (also called a prefix)](https://networkengineering.stackexchange.com/a/7117/16930) – JFL Nov 29 '17 at 12:48

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Yes, /21 is the largest supernet you can create containing all those subnets. You need to match the bits from left to right, completely correct.

Zac67
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