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Posted on 2014-02-08

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

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

2 Comments

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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