confusion in calculating Subnetworks

Unanswered Question
Feb 6th, 2007

Hi

i have read a question that you have class c address and you are borrowing 4 bits from the space then how much is the hosts available and networks available.

Most of the books says 14 networks and 14 hosts

some of the books says 16 networks and 14 hosts.

whats the answer is??

I have this problem too.
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scottmac Tue, 02/06/2007 - 18:18

Watch the phrasing of the question:

If it says "Total subnets" then the answer will usually be (for your example) 16 (four bits: counting 0 - 15)

If it says somethin glike "Total *usable* subnets" then the answer is usually 14 (four bits = 16, subtract two for the "unusable" subnets: 16 - 2 = 14.

For hosts in both cases, the "network address" (all zeros in the host portion) and the "broadcast address" (all ones in the host portion) are deducted from the total count.

Good Luck

Scott

Flash Weldon Mon, 02/22/2010 - 12:47

http://www.cisco.com/application/pdf/paws/13711/40.pdf

As of IOS 12.0, the "ip subnet-zero" command, default ON, allows the  use of the all-zeros and the all-ones subnets. Thus, a /28 bit mask creates 16 total as well as 16 usable subnets. It used to create only 14 usable ones (prior to IOS 12.0, default OFF). As far as the CCNA exam is concerned, the pertinent piece of information is whether "ip subnet-zero" is on or off (and they should show you enough of the  config to determine that).

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