subnetting - how to work out quickly VLSM's ?

Answered Question
Apr 19th, 2010
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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

Correct Answer by Jon Marshall about 7 years 1 month ago

carl_townshend wrote:


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


8 is the correct answer.


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

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Ganesh Hariharan Mon, 04/19/2010 - 07:18
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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

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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Correct Answer
Jon Marshall Mon, 04/19/2010 - 07:53
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carl_townshend wrote:


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


8 is the correct answer.


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

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