# subnetting - how to work out quickly VLSM's ?

Apr 19th, 2010

Hi All

say I have been given a /27 network

say 10.100.1.0/27

How do I quickly work out how many /30's I can fit into the /27?

As its 3 bit places to the right, do I just do 2x2x2 which is 8 or is this wrong?

cheers

Carl

`carl_townshend wrote:Hi Allsay I have been given a /27 networksay 10.100.1.0/27How do I quickly work out how many /30's I can fit into the /27?As its 3 bit places to the right, do I just do 2x2x2 which is 8 or is this wrong?cheersCarl`

Carl

I generally do it in my head like this

/27 = 255.255.255.224

/30 = 255.255.255.252 = 256 - 252 = 4 so /30 networks go up in 4

256 - 224 = 32 % 4 = 8

Another example - how many /29s are there in in a /25

25 = 255.255.255.128

/29 = 255.255.255.248 = 256 - 248 = 8 so /29 networks go up in 8.

256 - 128 = 128  % 8  = 16

Jon

Overall Rating: 5 (1 ratings)

## Replies

Ganesh Hariharan Mon, 04/19/2010 - 07:18
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`Hi Allsay I have been given a /27 networksay 10.100.1.0/27How do I quickly work out how many /30's I can fit into the /27?As its 3 bit places to the right, do I just do 2x2x2 which is 8 or is this wrong?cheersCarl`

Hi Carl,

Check out the below links on subnetting hope that helps !!

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800a67f5.shtml

and one handy tool on subnetting also

http://www.cisco.com/cgi-bin/Support/IpSubnet/subnets.pl

Hope to Help !!

Ganesh.H

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Jon Marshall Mon, 04/19/2010 - 07:53
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`carl_townshend wrote:Hi Allsay I have been given a /27 networksay 10.100.1.0/27How do I quickly work out how many /30's I can fit into the /27?As its 3 bit places to the right, do I just do 2x2x2 which is 8 or is this wrong?cheersCarl`

Carl

I generally do it in my head like this

/27 = 255.255.255.224

/30 = 255.255.255.252 = 256 - 252 = 4 so /30 networks go up in 4

256 - 224 = 32 % 4 = 8

Another example - how many /29s are there in in a /25

25 = 255.255.255.128

/29 = 255.255.255.248 = 256 - 248 = 8 so /29 networks go up in 8.

256 - 128 = 128  % 8  = 16

Jon