Vlan / Subnets

I've been reading up on VLANS on PETRI website and read the following statement.

"A common question is why not just subnet the network instead of using VLAN’s? Each VLAN should be in its own subnet. The benefit that a VLAN provides over a subnetted network is that devices in different physical locations, not going back to the same router, can be on the same network. The limitation of subnetting a network with a router is that all devices on that subnet must be connected to the same switch and that switch must be connected to a port on the router."

The bold bit above - I'm assuming when "Same Switch" is mentioned, we are also talking about interconnected switches not a single switch. I would have thought a subnet could span multiple layer 2 switches as long as they were all on the same side of the router interface?

Thanks

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Comments

fluk3d

A subnet can span multiple switches if the layer2 switch supports dot1q tagging/trunking. Layer3 switches wouldn't have any limitations as they generally do routing/switching and have 802.1q capabilities.

Usually when connecting two switches you would create a TRUNK between them so they could pass VLANID

martell1000

if you have different physical locatioons it can become tricky putting the switches into the same subnet. this is what the description is referring to.

SubnetZero

While it is possible to have Layer 2 only VLAN's, it's more common to have Layer 2 routed VLAN's for interVLAN communication. Back in the day before multilayer switches (MLS) existed the only way to route between your VLAN's was to use the router-on-a-stick approach.

That being said the statement made in bold is incorrect, here is from Cisco:

"A VLAN is a switched network that is logically segmented by function, project team, or application, without regard to the physical locations of the users."

In other words your VLAN's can span multiple switches just fine, the only requirement is that the VLAN's in question are allowed over your trunks.

In modern networks however the interVLAN routing occurs on the switch itself. For this to work you create a switched virtual interfaces (SVI's) for all of your VLAN's and the SVI becomes the equivalent to your router-on-a-stick.

HTH

Masternemo

Hello everybody. I ask maybe trivial easy question but I don't find a good answer for it:
Why we use VLAN ? Why? Why we cannot use ONLY IP-subnetting? When did appear REAL necessity for VLAN?
I read that we use VLAN for security, logical separation of users, separation one apartment's trafic from another apartment's trafic, inrcreasing number of broadcast domains, decreasing useless broadcast trafic and blablabla...
I feel that it isn't prime cause why human uses VLAN.

Please, give me the real examples when using of VLAN is very necessary and we cannot do without VLAN's help or it will cause big difficulties?

As I know VLAN was ISP's technology initially and only then VLAN has been used for small LANs.

Thanks

Roguetadhg

Why do we need vlans?

We don't need vlans. But they are a great help when used.

Vlans separate traffic, so User 1 can't talk to User 2 - through broadcasts, as well as communication. We have a strict HIPPA client. They require the use of VLANs to make sure there's no possible way to share information with other users on the network that aren't apart of the client.

Not to mention, it helps to cut some of the clutter and broadcasts that already have over 300 users in the same Broadcast Domain.

Masternemo

Yes, I good understand that vlans are used for decreasing useless broadcast trafic, but somebody tells me that vlan was ISP's technology initially.
Could you provide me some information about that?