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ICND1 subnet question


i am a bit confused about this. how do i go about this type of questions.

"network is assigned network manager requires 250 subnets. each subnet should support atleast 250 hosts.

what would be the subnet to use."

so in this type of questions how do i go about calculating the subnet? do i start by the required subnets or the hosts?

is there a simple calculation to do this?

3 Replies 3

Richard Burts
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The question that you pose has 2 constraints (the solution must have at least 250 subnets and each subnet must support at least 250 hosts) and the successful solution must satisfy both of them. I would suggest that you start with one constraint, find a solution that satisfies that constraint and then check to see if that solution also satisfies the other constraint.

I would probably start with the constraint about how many hosts are supported in various subnets. There is an easy chart that you can work out to provide this:

CIDR notation         subnet               usable

(/ notation)              mask                 hosts

    /30           2

    /29           6

    /28           14

    /27           30

    /26           62

    /25           126

    /24               254

From this chart it is evident that you need a subnet with mask (/24) to get the right number of hosts.

So then check to see if this solution also satisfies the other constraint. Since you were given a class B address, which uses 16 bits for the network part of the address, and you need to use 8 bits for the host part of the address,then it leaves you with 8 bits for the subnet part of the address. So how many subnets can you have with 8 bits for the subnet part of the address? The answer is just slightly more than 250.

So the answer to the question is that if you have network you need to create subnets with /24 ( to get the required number of hosts and required number of subnets.

I started by solving the constraint for number of hosts. You could just as easily start by solving for the number of subnets. And you should come out with the same answer either way.





You are given a network

And you have the requirement

    Size of Subnets: 250

    Size of Hosts: 250

Now calculate the size of each portion

NETWORK PORTION : 16 bits (as given 140.10)



It is always recommended to calculate the host portion first so that the remaining bits can be used for subnetting


SO we will calcuate the number of host bits required for 250 hosts-

(2^N)-2 =: 250

So for N=8 there will be valid 254 hosts

So the Host Portion: Last octet 8 bits


(2^N)-2 =: 250

So for N=8 there will be valid 254 subnets

So Subnet Poriton : 3rd OCTET 8 bits


For     N: Network Bits, S: Subnet Bits, H: Host Bits

Our Notation will be:



Subnet: to (having 250 subnets)

Host in each subnets will be:

    For Subnet , to (250 hosts)

    For Subnet to (250 hosts)

and so on.

Alexander Demin

Hello, Bhavesh!

A good explanation to this question along with other questions related to subnetting you can find in this book:

Chris Bryant (CCIE#12933). Mastering Binary Math And Subnetting. (

It's not an advertisment, It really helped me

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