VLAN and subletting together

[jorginho_monteiro]

Hi, this is my first post so I hope I'm doing it in the correct place, I've just started a CCNT course and I have a question, probably a silly one for you guys, can I subnet with VLANs ? I though the idea of subneting was kind of the same of VLANs, to keep networks in the same switch from accessing each other data, so, can I subnet with VLANs ? Kind regards Jorge

[paul driver]

Hello

On a L3 switch you can do this.

You can apply your subnetted ip range to each SVI ( switch virtual Interface) relating to the vlans you wish to use.

eg: 10.1.1.0/24

vlan100 - 10.1.1.0/25

vlan 200 - 10.1.1.128/25

int vlan 100
ip addres 10.1.1.1 255.255.255.128

int vlan 200
ip addres 10.1.1.129 255.255.255.128

res

Paul

[Martin Carr]

A VLAN is a logical network and should have it's own subnet.

Subnetting divides a larger network into smaller subnetworks (hence the name).

Martin

[devatt]

But on a L2 switch, must every VLAN have a unique subnet ? If so, how is this "enforced"? Does the IOS software or switch firmware limit this. Or is it as simple as at L3 devices on different subnets cannot communicate even if these devices are on the same VLAN?
 

[Jon Marshall]

But on a L2 switch, must every VLAN have a unique subnet ?

There is nothing that actually enforces this ie. you can have -

1) multiple L3 subnets within the same vlan using secondary addressing on the L3 interface. This is usually seen as a temporary measure when you are migrating a vlan from one IP subnet to another.

2) multple vlans using the same IP subnet. You see this usually when you need to deploy a firewall or load balancer in bridged mode where the same IP subnet is used on either side but you need to use different vlans on either side to avoid a L2 loop.

But the above are exceptions. The recommendation is to use one IP subnet per vlan. It makes everything a lot easier in terms of design and operations.

If you did have multiple subnets in the same vlan but no secondary addressing then yes as you say, communication between the two subnets would not work properly even though they are in the same vlan.

Jon