VLAN and subnetting

[neilneil]

I am really struggling to get going with this, hopefully I can explain it clearly enough.

A router has two interfaces of g/0/0/0 with an ip address of 90.0.0.1 255.0.0.0 and g0/0/1 with no ip address.

Two sub interfaces need to created on a router for VLANs 10 and 20. 

I have to put both VLANs along with another 16 subnets on a 172.16.0.0/24 network. 

Am i correct in saying that the VLANs and other subnets go on interface g0/0/1 with an ip address of, say, 172.16.0.1 255.255.255.224 for subnet 0 and 172.16.1.1 255.255.255.224 ?

Thank you so much in 

[Jon Marshall]

Not sure where the 16 other subnets come in but for the two main ones no it's not right. 

You need two /27 (255.255.255.224) subnets from 172.16.0.0/24 so you know straight away that the first three octets are going to be 172.16.0 (not 172.16.1 as you have used for your second subnet) 

To work out the actual subnets subtract the non 255 octet (224 in your example) from 256 which is 32 which means your subnets go up in 32's so your two subnets would be - 

172.16.0.0/27
172.16.0.32/27

Jon

[neilneil]

Hi Jon,

Thanks for replying.

This is the instruction i got "Using VLSM, split 172.16.0.0/24 into 16 further subnets. VLAN 10 will be in subnet 0, VLAN 20 will be in subnet 1. Create suitable interface IP addresses and subnet masks to reflect this configuration."

Many thanks,

Neil

[Jon Marshall]

Not sure what it is asking. 

Is that 16 subnets including the two /27s or in addition to ? 

Where does it mention the first two subnets should be /27s or is that something you decided ? 

Jon

[neilneil]

Hi Jon,

Sorry for the confusion, it does not say anywhere that the 1st two subnets should be /27, i just assumed this as these two subnets are in addition to the other 16 that is required to be created as well.

Hope you can understand what i am trying to convey, my technical knowledge is very limited, and i am very grateful for all help.

Neil

[Jon Marshall]

It's not really clear what problem you are trying to solve. 

Usually when asked to do this type of thing the requirement is for a certain number of hosts per subnet but you are just saying there should be 16 in addition to the two you have already mentioned. 

Also it is not clear why you assumed /27, as again there is no mention of how many hosts you need. 

Jon

[neilneil]

Hi Jon,

I thought the same thing regarding number of hosts. I think this is what has thrown me if i'm honest.

The reason i assumed /27 was because of there being 16 subnets plus the 2 vlan subnets. thus giving me 18 subnets and the last 5 bits totalling 33.

Thanks for all your help and taking time to look at this for me anyway, it is much appreciated.#

Neil

[Jon Marshall]

Yes those instructions are not clear at all so not your fault.

It could just be asking for you to create 16 subnets out of 172.16.0.0/24 and there are a number of variations but assuming equal size subnets then 256 / 16 = 16 so you could have - 

172.16.0.0/28
172.16.0.16/28
172.16.0.32/28
172.16.0.48/28
172.16.0.64/28

etc. 

ie. your subnets go up in 16s. 

But as I say the question does seem to involve some guesswork at best. 

Jon

[neilneil]

Hi Jon,

I thought it was going to be hard to VLSM without knowing the number of hosts per subnet. Just glad it wasn't just me not understanding something obvious.

I'll try not to bother you again, however, you have been fantastic and I'm glad of your input.

Neil

[MHM Cisco World]

172.16.0.0/24 must split into 172.16.0.X/27 not 172.16.1.X/27  
172.16.1.X/27 is different major subnet.

[MHM Cisco World]

@neilneil 
friend you use do hard work 
use any online subnet calculator 
and use it with any mask you want and see the number of user, 
then make 
subnet 1 have user start from this to this are this conflict with other subnet if yes then avoid it and go to other one.
until finally you get all subnet.