Answers: How Many Subnets/Hosts 1
Remember last Friday’s speed practice? Sorry for the delay in posting the answers. They are below the fold. As usual, I’ve listed the answers, but if it’s not clear why, you need to ask, so please do so. And sorry for the delay.
To answer these questions, you need to make two assumptions:
1) that the same mask is used for all subnets
2) that the zero subnet and broadcast subnet are allowed
Without the first assumption, you simply cannot determine the number of subnets. Without the second assumption, you cannot determine whether you need to subtract 2 from the total (if those two special subnets are not allowed), or not.
With those assumptions, all you have to do is determine the number of network, subnet, and host bits. You already know the prefix mask (/P) per each problem statement. From there:
a) Determine the number of network bits per class A, B, and C rules
b) Determine the number of subnet bits (S), which is the different between the prefix (P) and network bits (N): S = P – N
c) Determine the number of host bits, which is 32 – P.
Here are the answers to this set of questions.
Class  Network Bits (N)  Subnet Bits (S = P – N)  Host Bits (32 – P)  
1 
A 
8 
9 
15 
2 
A 
8 
13 
11 
3 
B 
16 
6 
10 
4 
C 
24 
3 
5 
5 
C 
24 
6 
2 

Number of Subnets 
Number of Hosts 
1 
2^9 = 512 
2^15 – 2 = 32,766 
2 
2^13 = 8192 
2^11 – 2 = 2046 
3 
2^6 = 64 
2^10 – 2 = 1022 
4 
2^3 = 8 
2^5 – 2 = 30 
5 
2^6 = 64 
2^2 – 2 = 2 
Hi,
Are we supposed to know the values of the powers of 2, or can we leave the answers in ex: 2^13 or 2^11 2. How will it be on the test? will there be calculators (if needed)? Thanks!
Hi,
Well, it’s impossible to know what might or might not be in a given question, but I’d say that knowing the powers of 2 is a good idea, say up through 2^16. It just comes up a lot in computing.
And… no calculators on the test.
Wendell
your answers can’t be right.
for example #1
Network=8
Subnet will be : 22 – 8 =14
Host will be : 32 22 = 10
Tem,
Not sure why you wrote 228 = 14. The mask in example 1 is /19.
Answers are correct as shown.
Hi Wendell, thanks for the answer. I wonder if it makes sense to subtract 2 from the calculated no. of hosts per subnet – doesn’t the condition “…that the zero subnet and broadcast subnet are allowed” imply that 2 does NOT have to be sutracted in this case (if I am right, then please note that I found the same issue in the CCNA vol. 1 book, let me know if you want me to give you the page no.)
Hi Gary,
There are two cases of the “2 or not” question with subnetting:
– Subtract two when calculating the number of subnets in a network
– Subtract two when calculating the number of hosts in a subnet
For the first one (# subnets in a network), you do not need to subtract 2. Many years ago, we did subtract two because in the old days we avoided using those two special subnets. No more. So, no 2 for # subnets per network.
For the second one (# hosts in a subnet) you always subtract two, as shown here and in the book.
I’m wondering if you’re mixing the two. Regardless, I believe it’s correct above and in the book. Feel free to follow up!
Wendell
Alright, that clarifies my confusion – I was indeed not aware of “2 or not” with no. of subnets in a network – thanks a lot!! =)