Subnetting question problem

Kaminsky

I'm fairly ok with subnetting but there are certain types of question I just can't figure out how to do.

You have a small network diagram and are given an IP address that has been assigned to your company by the isp. You have to pick from the available ip/sn masks answers provided and drag them onto the devices on the network diagram in the correct places to service X amount of hosts on that device.

The thing that is confusing me is that, apart from obvious wrong ip/sn to choose from, the given answers have different subnet masks on the IP addresses. ie xxx.xxx.xxx.yyy (/27 /30 /25 etc)

Surely once you have subnetted these would all need to be the same?

Could anyone give me any advice in how to answer these types of questions please?

loboernesto

what they want you to do is VLSM plan
I would first look for point-topoint connection between some routers. you have to choose the subnet numbers with the /30 prefix (be careful no to choose the subnet number instead of one of the two available host numbers in that case). For the rest of the interfaces you need to choose a subnet mask that will cover the number of hosts needed, start by working with the bigger subnets like this:

if router A ethernet0/0 interface has 28 hosts so you need a /27 prefix (255.255.255.224) that will give you 30 host . Then go to the next one for example router B ethernet0/1 interface that has 16 so you choose again /27 that give you 30 hosts. That one is a tricky one because you may think that with a /28 subnet you will get 16 hosts but remember the 2^n-2 rule which will leave only 14 hosts.

Hope that helps.

cheers

DirtySouth

It doesn't sound like you've studied VLSM yet. It basically allows you to create different sized subnets so that you're not waisting IP addresses. This way you can have a company with multiple different subnets.../27/28/29/30..etc

Kaminsky

Ahh.. Cheers Glad it's not my standard subnetting knowledge at fault

Why would you put a /30 on the serial interfaces ? I can only think of because it wouldn't take up many hosts so theoretically you would just need one or 2 for either end of the serial line?

Also, with vlsm, I presume you would look at the hosts required and subnet for the hosts of those ?

loboernesto

Yes you are right.
The whole idea of vlsm is to adjust the subnet so you only use one big enough to cover your needs so you can use or reserve the spaces for other part of your network. In a point-to-point only two addresses are needed so a /30 mask is perfect (it doesn't apply to all serial connections since you can have a frame relay in which case you need more than just two addresses).

I'm not sure if you have read vlsm yet, but once you do it you will get the complete idea. I remember seeing one question like the one you mentioned but I can't get an image of it in my head now, but doing as I said before you should get these questions out of the way in just a moment.

cheers

Kaminsky

I have read up on this now and gone through a load of examples writing down where all the ip addresses are used.

These questions take ages to answer and can be very sneaky especially regarding subnet 0's.

Is there any methods to spead these questions up or do you just have to go through tons and tons of examples on these?

loboernesto

these type of questions are big subnetting question involving all the other types of subnetting questions. Get quick with general subnetting questions and you will get quick with these ones as well. work every option out in the way you like. If you are just learning subnetting write every option down in binary including the subnet mask, this way you will memorize patterns and the relations between subnets mask in binary and in decimal. Once you got better you will be able to work out this question in your head. Consider taking up to 2 minutes maximum and try to speed up so you do them in 45 secs or 1 minute (if you take about 10-15 secs to work out the individual options you will be able to do this questions in minute or so).
cheers