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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 S

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..
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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.
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Hi Malmensa,

Thanks for the reply.

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

How many IPs do you require in each subnet?
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Andy S

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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 =
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?

6  buildings, lets just say there are 65 hosts per building and have been allocated
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
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.