Why does the decrease of the subnet mask double the original network size?

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Why does the decrease of the subnet mask double the original network size?

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#1 by (2 votes)

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Reading about IP protocol and about network addressing, I did not understand why the " decrease " of the subnet mascara size in 1 bit , generates a " > increase "of the size of the network.

I understood the reason for the increase, but not the fact that the decrease of 1 bit double the increase.

The reverse is also true from the source of ppgia .

Author'sAffirmation


Theprocessofgroupingsubnetsisequallysimple.Eachtimewedecreasethesizeofthesubnetmaskby1bit,wecreateasupernetthatistwicethesizeoftheoriginalnetwork.Inpractice,theconceptofsupernetworksisveryusefulfordecreasingthenumberofroutingtableentriesontheInternet.

source: ppgia.pucpr.br

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asked by anonymous 29.11.2017 / 20:22

1 answer

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score 2 · Accepted Answer

The mask is made to separate the part of the network from the part of the host.

The bits with value 1 represents the part of the network, while the bits with value 0 represent the part of the host. And these are bits with value 0 that are used to generate the unique IPs of the hosts.

This is worth explaining the notation / 24, / 25, etc. This notation counts the number of consecutive 1 in the mask.

The fact of increasing / decreasing 1% with% decreasing in half / doubling the size of the network is natural of binary representation. The size of the network is given by bit .

An illustrative example.

Mascara:    255.255.255.0 = 24  11111111.11111111.11111111. 00000000

So we have the last 8 2^(32-mascara) available, 2 ^ 8 = 256 IPs.

If we use the / 23 mask:

Mascara:    255.255.254.0 = 23  11111111.11111111.1111111 0.00000000

Now we have the last 9% bits available. 2 ^ 9 = 512 IPs. Double the previous one.