Find the "best" subnet

ch1vas

Hi guys,

I've been studying for 4 months now and almost ready to take the exam.

I have one big topic I'm struggling with... finding the best subnet.

I don't have any problems finding the subnet/host range/broadcast address, what I struggling are questions like:

"You need a minimum of 300 subnets and 50 hosts per subnet, which subnet would you take"

What do you find is the easiest approach on this? Right now I'm just calculating a 2 to the power of x and see where it gets me, then I change the x to y and see if that's any better.

Help appreciated.

mikespranger

i'm new too so don't take this as gospel, but this is how i would work it out. hopefully someone will come in and verify it for you (us)

for 50 hosts you need 6 host bits (62 hosts)
usually on questions like that they give you a class 2 address, so 16-6=10. with 10 network bits you can do well over 300 subnets (1024), so I would change it to 9 network bits for 512 subnets and 126 hosts per network, leaving you room for growth on individual networks.
7 host bits gives you a subnet of 255.255.255.128

iamme4eva

The Cisco exams are multiple choice, so the question would be more like "which of these subnets COULD you use (pick two)".

Calculate the host range and number of subnets for each answer, and see if it fits the criteria you've been asked for.

TehToG

I don't see why they couldn't ask "Which would be the most suitable subnet" and give two correct answers and expect the one which wastes the least space though.

iamme4eva

Wastes the least space how though? Depends on your forecast of how the network will expand - for the example above you could think "I'll choose 9 network bits cos I'm probably never doing to need 126 hosts", or you could plan for expansion and think "254 hosts would be nice, and I'll never need 256 subnets".

It's all a little subjective on your predictions of how the network may expand.

Maybe people who have done design stuff know better.