How many subnets do you have?.....

sonagccna

You are given a Class C network with 25 bits of networking. How many subnets do you have?
A. 1
B. 2
C. 3
D. 4

number of subnets = 2^no.of subnet bits or 2^no.of subnet bits - 2 ?

in particular which one is valid for the exam?

shadown7

You have 2 subnets here but none are usable with a class C address.


11111111.11111111.11111111.10000000

I've always used this formula when doing subnetting.

2n-2= (n) being the number of borrowed bits.

2(1) -2

2-2 = 0 usable subnets

I'd say the answer is (b.2)

dignity

u should able to have 2 subnets.. unless its not mentioned u can not use them..dont try to confuse your knowledge because there wont be any question like that.. ccna have better questions..

shadown7

dignity wrote:

..dont try to confuse your knowledge because there wont be any question like that.. ccna have better questions..

There is nothing confusing about it

nethead

There could be similar questions on the exam however my experience suggests that they will be better worded than this example.

sonagccna

the ans will be none, as the question asks how many subnets, so i can justify it to 2

and this Qn is from Richard Deal's CCNA prep. guide and Cisco press book says to get num. of subnets =2^no. of subnets - 2 (1.zero subnet & 2.broadcast subnet)

i follow cisco press book's way of subnetting

Drakonblayde

Well, you get 2 subnets by borrowing one bit. That much is true. You just don't get any usable (in the context of CCNA anyway) subnets. But, it didn't specify usable subnets, so the answer should be 2 to that question.

Bad question for a CCNA prep, IMHO

Jerz

Drakonblayde wrote:

Well, you get 2 subnets by borrowing one bit. That much is true. You just don't get any usable (in the context of CCNA anyway) subnets. But, it didn't specify usable subnets, so the answer should be 2 to that question.

Bad question for a CCNA prep, IMHO

But I thought in the '90's Cisco changed all of that where you no longer needed to subtract two subnets? It was my understanding that:
/25 = 2 subnets, /26=4subnets, /27 8 subnets, etc...

Webmaster

I think you should be able to determine from the details in the question whether you should subtract the 2 (ie. all routers have no ip subnet-zero configured), but whenever it isn't obvious, then you should subtract the 2. Though this contradicts with the current RFCs for subnetting which explains that the method from the older RFC (subtracting the 2 subnets) is obsolete.