subnet masks

[Arcaneuk10]

Question

You begin with a class C network ID of 192.168.1.0 and need to create 4 subnets with a maximum number of 25 hosts per subnet. Which of the following masks should you choose in order to meet the requirements? (Answer must leave room for future subnets

what I think the answer is.

255.255.255.192

the first 3 octets are the network part, so you cannot use them, the last octet has 2 subnet bits swiched on, .192 which equals to 2^2= 4 subnets, that leaves 6 host bits, I could switch on 1 more bit for future subnet bits, that would leave 5 host bits, which would be enough for 25 host bits as 2^5 is 32, is this acceptable, the other answers were

a)  255.255.255.0   b)  255.255.255.240.0   c)  255.255.255.192   d)  255.255.255.128  thank you for your patient, I am new to this.

regards

Arcaneuk10

[chrihussey · Accepted solution]

If you had a class C of 192.268.1.0 /24 and needed to create 4 subnets that would allow for 25 hosts in each and leave room for growth the answer would be to use a 255.255.255.224 mask which would provide 30 hosts each:

192.168.1.0 - 31

192.168.1.32 - 63

192.168.1.64 - 95

192.168.1.96 - 127

Leaving 192.168.128 - 255 available (or half of the Class C).

Didn't see that as a choice in any of your answers and not sure if this helps, but by standard methods that would be the best way to meet the objective.

Regards

[Arcaneuk10]

Many thanks, it has helped my understanding, very tricky question.

Regards

Arcaneuk

[Philip D'Ath]

I would run with 255.255.255.192.

[Arcaneuk10]

Thanks for your help, I think the key here is not to waste bits and follow best practices, it looks like this will have to be a hobby to make sure I understand it.

regards

[Julio E. Moisa]

That is correct, 

Please don't forget to rate or mark as answered the comments if they were useful

:-)

[Julio E. Moisa]

Hi

When you are subnetting there are to main formulas:

2^n >= # of subnets

[(2^n)-2] >= # of hosts

Where "n" represents the amount of bits substracted from the available bits to subnetting. 

If you have a request with X number of subnets with X number of hosts, ¿what is the priority? the priority will be X number of hosts because you cannot have subnets with other amount of hosts, so you prefer the second formula to get that hosts and subnets:

[(2^n)-2] >= # of hosts

So if your request is: 

need to create 4 subnets with a maximum number of 25 hosts per subnet from the parent subnet 192.168.1.0/24

You have to do:

 Available bits to subnetting:  Total of bit of IPv4 - CIDR of the parent subnet

32 - 24 = 8 bits ; 8 available bits to subnetting

Next step is verify how many bits you need to get 25 hosts. If n = 5 the result is:

[(2^n)-2] >= # of hosts

[(2^5)-2] >= 25 of hosts

32 - 2 >= 25

30 >= 25 

The next step is get the new subnet mask

We have 8 bits available and we substract 5 from right to left

1111 1111 = 3 bits

So your mask should be 255.255.255.1110 0000 = 255.255.255.224 = /27

Next step is get the networks

256 - modified octet

256 - 224 = 32

So your networks are:

192.168.1.0/27

192.168.1.32/27

192.168.1.64/27

192.168.1.96/27

192.168.1.128/27

Your should have 255.255.255.224 as answer but none of the mentioned subnet masks are correct so you should select none of them. Because the correct ends in 224. 

Your could select 255.255.255.192 or 255.255.255.128  but they will waste many IP addresses and that is not the target of subnetting, no a best practice. 

Hope it is useful

:-)

[Arcaneuk10]

Hi

Very informative and helpful, I am aware that you must not waste bits, and best practice is very important, I suppose that is the best way to answer the question based on that. Thnks for your help.

[Julio E. Moisa]

Hi

It was a pleasure.

Have a great day

:-)