Unanswered Question
Apr 9th, 2007

I am new to Cisco and I am studing for the CCNA exam. I am slightly confused on when to use the 2^?-2 and when not to. I found this on the Cisco website, "On the issue of using subnet zero and the all-ones subnet, RFC 1878 states, "This practice (of excluding all-zeros and all-ones subnets) is obsolete. Modern software will be able to utilize all definable networks." Today, the use of subnet zero and the all-ones subnet is generally accepted and most vendors support their use. However, on certain networks, particularly the ones using legacy software, the use of subnet zero and the all-ones subnet can lead to problems."

But several of the study books say use the 2^? -2 to find subnets. Can anyone clear up when I am suppose to use the -2 and when not to.


I have this problem too.
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punkascii Fri, 04/13/2007 - 11:36

Do exactly what the question asks. If it doesn't specify it will be using the 0 subnet, observe the -2 rule.

2pparish Fri, 04/13/2007 - 20:10

But the INTRO CCNA book by cisco press. There are a few good questions on the CD that relate to this. One had to do with EIGRP and being a classless routing protocol. I think it has to do with classless and classful routing protocols and how they exchange routing updates.

-2 is for subnet zero and broadcast subnets.

thsinghosn Sat, 04/14/2007 - 01:18

in the exam,you should use -2 networks

in realworld networks,you can use subnetzero net


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