VLANs for the Homelab: A beginner's guide to segmenting networks

I tried to condense the countless hours I spent learning what VLANs are and how exactly to implement them, trying to focus on the important information.
What is a VLAN?
A VLAN is short for Virtual LAN. A VLAN is an isolated broadcast domain. If that doesn't mean anything to you, we are able to just call it an isolated segment or isolated section of a network, where devices on that segment cannot "see" devices on other segments.
One way which has helped me to grasp this concept is thinking about mDNS or DLNA devices (another little rabbit hole to decrease, but for now stick to me). Look at a wireless printer or perhaps a Chromecast. The unit usually use technology that allows them to "magically" appear on your phone if you're connected to the same network. That is because your phone (or laptop) and the Chromecast or printer come in exactly the same broadcast domain or segment. If your laptop as well as your Chromecast were each in different segmented VLANs, it could be as though the Chromecast doesn't exist so far as your laptop is concerned.
Finally, a VLAN is not a subnet. That is imporant, we will touch on this in a future post about inter-VLAN routing.
So it breaks my Chromecast. Why would I want to use it?
While in the case of attempting to print to a wireless printer from your work laptop, having each device in another network segment will be annoying, there are several valid use-cases for VLAN segmentations in the homelab or home newtworking setups, including:
Isolating work-from-home devices (such as a work laptop, printer) from personal devices
Isolating "production" servers from "staging" or "development" servers if you are running some type of application on your homelab
Isolating IoT or untrusted devices - for instance having all of your Alexa or smart home devices connected on an isolated VLAN so they can't "see" and spy on your own internal network
Ability to finely control inter-VLAN routing - that is one we will touch on later on, as VLANs allow a network administrator to establish rules for how different VLANs can interact with each other and with the web - for instance a "kids" VLAN it doesn't have access to a specific game after 10PM
VLAN-aware Switches and Routers
Before we move around in to more details about implementation, I want to briefly discuss the hardware side of things. This will be an extremely high-level overview as there are plenty of resources for learning the actual "behind-the-scenes" of how VLANs work in networking hardware. I'm aiming to ensure it is easily understood for a newcomer or beginner.
Remember: Part of setting up VLANs is learning the quirks of your particular equipment and not to assume one vendor will undoubtedly be like another in their VLAN implementation. You will see different manufactures will have slightly different implementations, but the overall concept remains the same!
Why can't all routers and switches support VLANs, isn't it just a software thing?
Yes and no. By the end of your day, all a VLAN really is in practice is a tiny bit of extra information added to every "packet" of information traveling during your network. This information needs to be interpreted and treated accordingly by your equipment. This could be done in software or hardware. BUT, remember that our routers and switches are usually not very powerful with regards to software tasks.
Network switches are low-power, efficient devices that do a simple job really efficiently with hardware. That is why a Mikrotik CRS-328 can switch 63gbps of traffic when it includes a single 800 mhz CPU. When it needs to utilize its CPU to route traffic, like routing traffic from one VLAN to some other, that throughput number falls to under 500 mbps.
There are 3 solutions to this problem:
Throw more power at it: a powerful CPU can handle VLAN tagging without much issue - think about a Proxmox or other virtualization server, that may handle VLAN traffic
Use hardware that is optimized for the duty: find a "managed" switch that has Layer 2 capability if you just need simple VLAN capability or Layer 3 Hardware capability if you'd like the switch to be able to route traffic between different VLANs
Be OK with lower speeds or decreased efficiency. That is sometimes the answer, especially for the homelab. For example, my travel router, a GL-AR750s has OpenWrt installed and can do VLAN filtering through software. I'm fine that this isn't the most efficient setup possible and value the convenience and cost effectiveness more in this example. This may connect with many beginners, who have a router that can just be flashed with OpenWrt and handle VLANs through its CPU. It's still exactly the same VLAN goodness, just slower and cheaper!
VLAN terminology glossary
The following are some important terms and concepts to understand. These can help you grasp diagrams and types of VLAN setups and translate them to your personal equipment:
VLAN ID/VID:
This is actually the number, 1-4095 of the VLAN. This is used by networking equipment to identify and group members of exactly the same VLAN together. It is very important realize that the quantity is all that matters. If you label VLAN 10 "Sally" on one switch and VLAN 10 "Jimmy" on another, the only thing the switches really value is the VID
Tag
poe splitter
A VLAN tag is a little piece of information added to a packet that tells networking equipment which VLAN that packet belongs to.
Tagged
When setting up a VLAN-aware switch, you'll often come across the term tagged or untagged in the form of a checkbox or dropdown menu for every physical port on the switch or router (and sometimes the "CPU" is known as its own port, like in the case of OpenWrt or "bridge" regarding Mikrotik - this is very important to inter-VLAN routing which is discussed later). When choosing "tagged" - you are indicating to the switch that the traffic on that port with the VID you're marking as TAGGED should keep its VLAN tag when it leaves/enters the switch.
In case a VLAN is tagged on a port and you connect a non-VLAN aware device, that traffic will be invisible compared to that device, while a VLAN-aware device will be able to pick up that traffic and filter it. This is ideal for trunking (also defined in this list).
Untagged (access port on Cisco)
The inverse of the aforementioned, you're telling the switch that traffic with the untagged VID will have its tag removed as it leaves the switch, so that it will show to the connected device as though it is regular non-VLAN traffic.
PVID
This is actually the physical port VID. It is possible to think of this because the "default" VLAN ID for the port, meaning that packets coming into/leaving the switch with no tag will be considered to be part of this VLAN. You could be confused by the point of the when you likewise have untagged/access ports. On Mikrotik if you decide on a PVID for a port it'll automatically show that VID as untagged on that port, without you needing to manually add it. On my TP-Link T1600G I had to choose a PVID for a port and choose a port as untagged. From what I could tell reading on forums, some vendors separate egress/ingress via untagged/PVID respectively. Just know to make certain together with your documentation which pattern your equipment follows.
Trunk
This is one of the best and what I think may be the most powerful parts of VLANs, trunks. A trunk is formed once you tag multiple VIDs about the same port on one switch. This port may then be used to connect another VLAN-aware switch and be able to use the same VLANs on that switch too. An extremely real usage of this: I have 2 VLAN-aware switches in my network: a Mikrotik CRS-328 which is connected via a trunk port to a Mikrotik CRS-317. This enables me to use extend my network and have the excess 10G SFP+ ports on the next switch, while still keeping exactly the same network segmentation.