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My understanding of VLAN must be flawed because it's not making any sense to me and seems redundant.

So lets say I have two IP ranges: 192.168.0.1-192.168.0.254 with subnet 255.255.255.0 192.168.1.1-192.168.1.254 with subnet 255.255.255.0

Now, from these two ranges, a single router is used. Each IP range is on a separate LAN port of the router. I am asserting that these are two completely separate broadcast domains because the routing table has entries between these two IP ranges to allow routes between them (if I remove or disable these entries then they are unable to communicate). [If my assertion here is wrong please let me know].

Assuming the above is correct, now enter two VLANs VLAN1 and VLAN2. Now, my understanding is that a VLAN creates two or more broadcast domains. So in my mind I would configure VLAN1 on 192.168.0.0 and VLAN2 on 192.168.1.0 but isn't that just redundant since these two IP ranges are already separate broadcast domains? So why would I choose to VLAN tag them? What am I not understanding?

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  • So I ask a legitimate question and I get a downvote?
    – Quilnux
    Nov 10, 2017 at 12:34
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    Wow - so out of your depth - as we make very clear when you sign up this site is for professionals, not learners, please read and adhere to our help pages and the workings of the site before posting again.
    – Chopper3
    Nov 10, 2017 at 18:32
  • It was a professional question. It's related to my job. But ok. If the site isn't for "learners" then it's existence would not make since considering that asking a question is to gain knowledge (learn) about the topic being asked. So if we aren't suppose to learn about a topic being asked then what is the site for since asking questions is out according to you.
    – Quilnux
    Nov 11, 2017 at 21:41
  • For your consumption: If your question is about: managing the hardware or software of servers, workstations, storage or networks tools used for administering, monitoring, or automating these deployment to and management of third-party provided information technology platforms Since this is regarding managing networks this would be well within the rights of the rules.
    – Quilnux
    Nov 11, 2017 at 21:44

3 Answers 3

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VLAN's are a Layer 2 construct. Routing is a Layer 3 construct.

VLAN's provide "separation" at the data link layer (Layer 2).

A Layer 2 broadcast or a Layer 3 broadcast will be flooded by the switch to all ports in the same physical broadcast domain, which means all ports in the same VLAN. A Layer 3 broadcast is meant for all hosts in the same Layer 3 network and devices not in that network will ignore this traffic, but the switch will flood this traffic nonetheless as the Layer 2 address for both types of broadcasts is the same (FF-FF-FF-FF-FF-FF). See the attached images of broadcast packets for clarification. You can see that the Layer 3 destination address in both packets are different (one is a Layer 3 broadcast and the other is a Layer 2 broadcast), but the Layer 2 destination address is the same (FF-FF-FF-FF-FF-FF). Both of these packets will be flooded by the switch to all ports in the same physical broadcast domain). Again, one reason to implement VLAN's is to "separate" this traffic.

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So while the different subnets provide logical separation at Layer 3, they provide no separation at Layer 2. This is one reason for implementing VLAN's.

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  • Ok. I get it now. I was not aware that broadcast was possible at both L3 and L2. I always thought it was an L3 thing so this makes total since. thanks!
    – Quilnux
    Nov 8, 2017 at 12:51
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    Just a minor detail, I would use term "link layer" instead of "physical layer" in the second paragraph. Nov 10, 2017 at 18:19
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VLANs (Virtual LAN) are defined on the L2 layer of the ISO OSI model. It can be used for logical separation of the physical topology.

There is more complex description on the wiki:

https://en.wikipedia.org/wiki/Virtual_LAN

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  • Unfortunately that doesn't even come close to answering my question. I already know they are Layer 2 of the OSI model. That's not the question. My assuming that my first IP range should be tagged with VLAN1 and my second IP range should be tagged with VLAN2 came from me reading that Wiki page. This made my wonder why I would do this redundant action and question why VLANs would exist in the first place.
    – Quilnux
    Nov 8, 2017 at 12:24
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Each IP range is on a separate LAN port of the router.

Probably that router already has internal VLANs as the different subnets are on separate ports. Now imagine you wan't to share this 192.168.0.0/24 on port LAN1 to one group of client computers and the 192.168.1.0/24 on port LAN2 to a different one.

  • Without VLAN tagging (or with unmanaged switches) you need one switch for every network.

    If you try to use the same unmanaged switch for two networks you end up having problems when a client gets different IP depending on witch DHCP server answered first (unless it's the same server with reservation for every client). Also, the clients may see traffic from other subnets or move freely between them by changing their IP address.

  • With VLAN tagging you can use a single port between the router and managed switch for all the L2 networks: the tagging expands the VLANs between network devices. Then, the ports connected directly to your client devices have only a single VLAN untagged, making all the ports with the same untagged VLAN act like they were a single unmanaged switch.

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  • I didn't really think about the physical connections related to VLAN as my question was only considering the broadcast domain aspects but this actually helped because I'm in the middle of replacing network equipment for a client and I was just about to purchase two switches but with this I am reconsidering buying only one. Thank you!
    – Quilnux
    Nov 8, 2017 at 13:16

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