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Mnemonic to calculate Supernetting

Posted on 2014-02-08
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Last Modified: 2014-02-16
Hi

I must teach our trainees how to calculate Supernetting by mental arithmetic as they need it for School.
I leaned this a long time ago but have forgotten this as this is used very rare.

Can you please explain me the best way how to calculate this stuff and tell me about your mnemonic’s?

Many thanks in advance
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Question by:*** Hopeleonie ***
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Fred Marshall earned 2000 total points
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Well, I don't know if I'd necessarily use the term "supernetting" as it may connote things not intended.  I'm hoping that you're asking about the following:

Given: an IP address and subnet mask 192.168.1.33/24
With /24 or 255.255.255.0, the "33" is the host address and 192.168.1.0 is the network address.

Question: What is the next (i.e. smallest) larger subnet that will include the address and subnet of the given address?

Answer: Remove the least significant "1" bit from the subnet mask and see:

255.255.255.0 decimal is 11111111 11111111 1111111 00000000 binary.
So, we use:
11111111 11111111 11111110 00000000 or 255.255.254.0
(since in the third octet, 255 -1 = 254)
and, if you wanted to go to an even larger subnet you would remove the two least significant "1" bits for:
11111111 11111111 11111100 00000000 or 255.255.252.0 (255-3 = 252)

The network address changes as follows:
If you remove the least significant "1" bit from the subnet mask, you also remove the least significant bit from the network address you're staring with.  So:
192.168.1.0 is
 11000000 10101000 00000001 00000000
Removing the least significant "1" bit gives
11000000 10101000 00000000 00000000
or 192.168.0.0

I don't know if that's a mnemonic but I hope it helps...
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by:*** Hopeleonie ***
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@fmarshall
Thanks a lot. Learned a lot from you :-)
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