Subnetting Question

danb83

Can anyone confirm that this is correct:

A national retail chain needs to design an IP addressing scheme to support a nationwide network.
The company needs a minimum of 300 sub-networks and a maximum of 50 host addresses per
subnet. Working with only one Class B address, which of the following subnet masks will support
an appropriate addressing scheme? (Choose two.)


A. 255.255.255.0
B. 255.255.255.128
C. 255.255.252.0
D. 255.255.255.224
E. 255.255.255.192
F. 255.255.248.0


Answer: B,E

I am thinking that there may be an error and that the question should be a MINIMUM of 50 hosts per subnet?

B: /25 allows for 126 hosts
E: /26 allows for 62 hosts

Hondabuff

When you are subnetting, you are looking for block sizes. 2,4,8,16,32,64,128,256,512 and so on. Think of the block sizes as buckets. A 64 bucket can only carry up to 64 hosts, if you want to carry 65 hosts you will need the bigger bucket "128". So the thought is, Don't forget the N-2 equation for finding hosts. 64 block is actually 62 hosts. I need a bucket that can carry 50 hosts without going over the cap. A 32 bucket would spill over but a 64 bucket will carry 50 but have some leftover space.

Jon_Cisco

The question has two halves that you need to consider.

Class B

N.N.H.H so you have 16 host bits.
To subnet the hosts for 300 networks you need to go to the 512 bucket. 9bits.

This gives you 7 bits to play with for hosts. N.N.ssss ssss.shhh hhhh

Now you need to fit 50 hosts in your bucket.
8 bits 256 (already taken for subnet)
7 bits 128
6 bits 64
5 bits 32
4 bits 16
3 bits 8
2 bits 4
1 bit 2

So your appropriate buckets are 7 bits and 6 bits.

Convers to subnet mask

11111111.11111111.11111111.10000000
255.255.255.128
and
11111111.11111111.11111111.11000000
255.255.255.192

Heero

danb83 wrote: »

I am thinking that there may be an error and that the question should be a MINIMUM of 50 hosts per subnet?

They either mean minimum of 50 hosts per subnet, or that the maximum number of hosts that will need IPs on the subnet is 50 and the subnet size needs to support it.

The Reluctant Loser

What they mean in this example is that there will never have a need for more than 50 users in that network. That question confused me a bit when I was taking ICND1 as well. Jeremy from CBT Nuggets has some great videos about subnetting that will help you with this style of question.

mella060

Well B and E are the only answers that meet the requirements. If you need 300 subnets, then you will need 9 bits because 2^9 = 512. 8 bits would only give you 256 networks. 2^8 = 256.

For 50 hosts then you will need at least 6 host bits 2^6-2 = 64-2 = 62 hosts. Answer D only uses 5 host bits which only gives you 30 usable hosts per subnet. 2^5-2 = 32-2 = 30

Subnetting is all about knowing the subnetting formulas for networks (2^) and hosts (2^-2) and the powers of two.

pevangel

You really shouldn't be using **** as study materials. It violates the exam agreement and it is cheating!