Can't quiet figure out this one subnet question.

Raymond Mason

Hello! I have been practicing on subnet questions for a few days now and getting a hang of it. Although there is one question that threw me off:

Question: You are designing a subnet mask for the 172.16.0.0 network. You want 30 subnets with up to 1400 hosts on each subnet. What subnet mask should you use?

The thing is I can't get exactly 30 subnets or exactly 1400 hosts on each subnet. Do they just want a subnet ball park that can fit all of that? I am thinking I can do this with VLSM but I haven't dived into that yet.

Queue

30 subnets 2^5=32
16+5= 21
32-21= 11
2^11=2048 -2 =2046
Fixed mask.
16+5=/21
I did just get 50% in IPv4 on my exam, but I'm pretty sure thats the answer.

Raymond Mason

[Unnecessary quote removed]

I came up with the same answer as you but I wasn't sure if they wanted a ball park or what was the deal.

Nans

@=Raymond Mason]

Don't get confused. It clearly said upto that means that's the minimum requirement they asked. So what you could have done there is utilize all the address, because of which you will get 32 subnets (You need not do VLSM since they didn't ask different host count for different subnets) and 2046 hosts in each subnet. The mask would be 255.255.248.0

I don't know what you meant about ball park...!!

clintonia

[Unnecessary quote removed]

I think what the OP meant by "ballpark" was do they want an estimate that's close to the requirement.

@OP, to answer your question, yes, they want a ballpark. If they ask for 30 subnets, you can't get exactly 30 subnets so you'd have to go with 32 which exceeds what they're asking for, however it gives you at least 30 subnets, which they are looking for

Also, wouldn't the subnet mask be 255.255.248.0?

HP_Guy

To be pedantique, if they'd asked for 34 subnets, then 32 would still be close and in 'the ballpark', but would be WRONG!

Priston

I agree, they don't want something in the ballpark. They want something that meets both requirements. 30 subnets, 1400 hosts.

4 subnet bits gives you 16 subnets and 4094 hosts, while this meets the host requirement it does NOT meet the subnet requirement.
5 subnet bits gives you 32 subnets and 2046 hosts, this meets both requirements.
6 subnet bits gives you 64 subnets and 1022 hosts, while this meets the subnet requirement it does NOT meet the host requirement.

pinkiaiii

/21 and 255.255.248.0
seems answer you would need since,not even counting subnets if you look at amount of hosts required you need to pick at least 2048,thus going by curriculum its always better to reduce wastage of IPs,but its also ok to have spare addresses for expansion,while as said /22 would suit ballpark but it would amount not meeting criteria of 1400 per subnet-thus would be incorrect,as opposed having extra addresses which is reasonable,and satisfies both criteria.

Nans

clintonia wrote: »

Also, wouldn't the subnet mask be 255.255.248.0?

Oops my bad, I apologize for that.. Thank you for correcting me on that...