IP VLAN Overlapping

Hi All,

I've been asked to setup a network with 9 VLANS and have been allocated an IP address of

I can create the first VLAN '10' as
When I go to create VLAN '20' as

I receive a overlaps with Vlan '10'

I can only then add VLAN '20' as, although if I'm only allocated '' would this still work? What would be best practice?

I'm using a CISCO 3960 Switch
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Andy SCommented:
If you create your first VLAN '10' as it's essentially using the IP's from -
You'll need to break down the /21 you have into smaller subnets, The number of hosts you require in each network might help you determine the exact split. some might be small subnets, others might be larger..
Mal OsborneAlpha GeekCommented:
There are a heap of ways this could be done. In the name of simplicity, I would probably use a /16 mask, and use the 2nd octet to select the VLAN.

ie,  = VLAN100 = VLAN200 = VLAN300.

That gives you 256 VLANS, each with 64K IPs.
logicsolutionsAuthor Commented:
Hi Malmensa,

Thanks for the reply.

I've only been allocated What cab we do to work around this solution?
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Mal OsborneAlpha GeekCommented:
OK, I misunderstood.

How many IPs do you require in each subnet?
Andy SCommented:
You won;t get an even split, but if you did them as /25, you get as below, you could throw a couple of /24's in to to give you a few bigger subnets,..It really depends how many devices you need in each, or if there's a physical layout that might help, for example if splitting it by locations, one might be big so give it a /24

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logicsolutionsAuthor Commented:
Thanks for your input Andy - Mind if I explain something to you here?

Say there was 6 Buildings, 1 switch and I've been allocated

Would you then setup each building as the following ?

Building 1
Building 2
Building 3
Building 4 =
Building 5 =
Building 6 =
Mal OsborneAlpha GeekCommented:
Nope. is a block of 2046 useable addresses, from to  

If you want to keep subnets within that range, you need to implement some smaller subnets.  

As Andy said, you COULD use /25, and have 16 subnets with 126 useable addresses each. /24 would give you 8 subnets of 254.

If you have different numbers of hosts for each subnet, then it is possible to use different masks.

How many hosts are required in each subnet?
logicsolutionsAuthor Commented:

6  buildings, lets just say there are 65 hosts per building and have been allocated
Mal OsborneAlpha GeekCommented:
65 is a difficult number.   A /26 would give you 62 hosts per subnet.

Using /25 you can get 16 subnets of 126 useable IPs each.

Andy nearly had it.

Your actual subnets will be
Don JohnstonInstructorCommented:
Actually, your subnet addresses would be:

Unless you know that you'll never have more than 62 hosts per subnet, I would go with a /24.

Each subnet can have 254 hosts and you've got two unused subnets.
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