Calculating subnets

Unanswered Question
Mar 25th, 2007

Hello,

I'm really just after confirmation of a change in formula. I have heard that the formula for calculating the usable subnets is now 2^n not the 2^n -2 as it was previously?

Therefore is this correct for the CCNA exam? Also is this the same for usable hosts and all the other formulas that have the -2 portion?

Cheers

I have this problem too.
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Danilo Dy Sun, 03/25/2007 - 02:46

To calculate the number of subnets or nodes, use the formula (2^n-2) where n = number of bits in either field, and 2n represents 2 raised to the nth power. Multiplying the number of subnets by the number of nodes available per subnet gives you the total number of nodes available for your class and subnet mask. Also, note that although subnet masks with non-contiguous mask bits are allowed, they are not recommended.

Step-by-step tutorial from http://www.ralphb.net/IPSubnet/ just select "Next" to go to next page.

Subnet calculator from Boson and other utilities http://www.boson.com/FreeUtilities.html

IP Subnet calculator from Cisco and other utilities if you have CCO Account http://www.cisco.com/en/US/support/tsd_tools_by_category.html

swmorris Sun, 03/25/2007 - 09:42

Sorry for the correction... But a subnet cannot have discontiguous bits. That would make life WAY too confusing.

ACLs can use discontiguous bits, but save that for later lessons!

The "-2" part is for the network ID (first IP) and broadcast IP (last IP) of any range. These are still required in IPv4 except when using a /31 mask.

That /31 is a special exception, and should not be looked at when answering questions at the CCNA level.

HTH,

Scott

smorris@ipexpert.com

cameron.moody Sun, 03/25/2007 - 21:32

Hi,

Thanks for that. I guess then I am querying the formula as stated for the CBT that I am going through (trainsignal ccna). Because of that I am wondering if the change applies to any other formulas.

On their "corrections and updates" it states

"Since the release of this video Cisco has modified the equation for calculating the number of subnets on a given network. The number of subnets is now calculated as such:

Number of subnets = 2?n where n = the number of subnet bits."

http://www.trainsignal.com/index.asp?PageAction=Custom&ID=34

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