CCNA Version 2 Question 1

[netwalker1]

Given a class B IP address with a 20 bit mask what is the maximum useable subnets ?

My answer was : because class B address comes with 16 bit subnet mask by default , so there is 4 bits for the subnet address , which make the subnet numbers is 2^4=16 subnets ( as CCNA Ver 2 )
but the answer was 14,, they substract 2 from my answer as there is 2 subnets reserved ! but I think this was only in the CCNA Ver 1 but as a CCNA ver 2 which use the zero-subnets ..
Is my answer true or false ?!!!!!

 

[wybnormal]

When I took version 2 right after it came out, Zero subnet was illegal to use... hence you subtract all zeros and all ones to get the correct answer. The caveat with zero subnets is ALL layer three devices on the net needs to be able to use subnet zero and many devices dont allow it.

I will say I have heard much debate about it... all my practice tests (so far) for the CCNP also want to see you whack two subnets off the total to get the correct answer unless it *specificly* calls out subnet zero as being legal in the question/case study.

Just my experience so far.

Mike S
"Diplomacy; the art of saying 'nice doggie' till you can find a rock" Wynn Catlin

 

[netwalker1]

Well,, Thanx wybnormal ,,
But in the ( Cisco CCNA Exam #640-570 Certification Guide )
- which was published by Wendell Odom (year 2000) - I found a note which says "Don't care about the reversed subnetworks" and there are some examples which the author didn't subtract the 2 reserved networks !!
but in my practices about it ( which are mainly from Bosen " I found myself forced to substract them !!
I wounder if you have any advices !

 

[talisker]

Wybnormal is right about all devices not supporting zero subnets.

I spoke to a couple of Cisco instructors & they say that the formula is (2^n)-2 for both CCNA v1 & v2.