Look at them like this: VLANs are like running several cables between two spots that you can configure independently. In the very end it comes down to this: what virtual LAN number you have on the cable.

Your backbone devices (router and switches) can be configured to accept tagged traffic―your switch will send a packet prefixed with a VLAN index and your router will trust that the packet actually came from that VLAN on the switch port, or to tag traffic―like when you have some port on your switch where your PC is plugged in and the switch will tag those packets with some VLAN when it forwards them (to the router).

Once you grasp that, everything else pretty much boils down to managing several isolated networkd and how they cross-talk. You run a dhcp server over each network, its own set of other services and whatnot.

Oftentimes the “home” hardware will expect a single network and use some means of packets broadcast to reach each other. That's how your phone can find all google homes on the network and apple homekit knows where your smart lights are. For that traffic to cross VLANs you’ll have to use some special software like mdns repeaters, but you can still isolate them.

Wrapping up, VLANs basically allow you the physical level isolation over a single cable. Mind that there are, of course, some bugs, e.g. I once found an issue with Unifi access points that allowed a well crafted packet to escape into VLAN 1 no matter what it was supposed to be tagged with. So don’t treat them as physically separate links.

They are not hard once you grasp the idea. They are like separate networks on layer 2(link) - layer 1 (physical) can be shared.
So you get several separate networks for the price (and equipment) of one. If you want to reach a device on one vlan from another it needs to be forwarded by something.

It gets a bit complicated here - as your idea of the network is on layer 4 where tcp and udp and other protocols live. As you don’t want to connect one vlan to the other - you want something that has access to both vlans to forward your layer 3 data (IP) between the links. This is your router. It will have a virtual network card on each vlan. You can tell your router to send data from one network card to the other to forward the data.

I suck at explaining- so you probably better off doing an Udemy network primer or read up a little bit. Good things to understand are the first 4 layers of osi model and routing.

It’s not hard and you can learn how to use it by poking stuff and googling a bit. Just imagine each vlan as a “copy” of your equipment (layer 1) cables and all. Your switch will have to support it, and if you want to trunk (run several vlans though one link) you need support on the other end as well.

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