5

With VLANs I can connect two computers which are:

  • in the same subnet
  • in different subnets

And I can make two networks plugged into one switch (S1), so: having on the left side of S1 the network 192.168.1.0/24 VLAN 11 and on the right side 192.168.2.0/ 24 VLAN 22. That would also work without VLANs, because all ports don't have to be in the same LAN. (There can be multiple networks hanging on one switch)

This all due to the oparation of the switch at layer 2. So, he doesn't care which subnet the fame is in, because the switch only cares about the VLAN-ID/MAC-Adress.

Is my assumption right? Please correct me if I got something wrong.

5

Each VLAN connects nodes into a single segment/broadcast domain(/subnet). Think of a VLAN as if you were using an additional switch with no connection to your main switch.

The difference to that scenario is that VLANs can use tagging on a link and you can use this link to carry multiple VLANs.

  • 1
    "with no connection..." - That oversimplification can lead you down the garden path. It is true with switches that use a per-VLAN CAM table (eg 3com derived HP), but has interesting exceptions with switches that do not (eg many procurve derived HP). In the latter case, things you would expect to work (eg externally linking two port based VLANs with a patch cable) surprisingly WON'T. The same applies to differences in how a particular switch handles spanning tree. – rackandboneman Feb 17 '18 at 20:44
  • Yes, this is simplified, I'm trying to convey the general idea. I don't think specific implementation quirks are important here. (Although I've seen the exact VLAN relabeling issue you're talking about ;-) – Zac67 Feb 18 '18 at 9:45

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