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 to implement them, trying to concentrate 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 part of a network, where devices on that segment can not "see" devices on other segments.
One way that has helped me to grasp this idea is thinking of mDNS or DLNA devices (another little rabbit hole to go down, but for now stick with me). Think about a wireless printer or a Chromecast. The unit usually use technology which allows them to "magically" appear on your own phone if you're connected to the same network. That's 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 another post about inter-VLAN routing.
So it breaks my Chromecast. Why would I want to use it?
Within the case of attempting to print to a radio printer from your work laptop, having each device in a different network segment would be annoying, there are numerous valid use-cases for VLAN segmentations in the homelab or home newtworking setups, including:
Isolating work-from-home devices (just like a work laptop, printer) from personal devices
Isolating "production" servers from "staging" or "development" servers if you are running some form of application on your homelab
Isolating IoT or untrusted devices - for example having all of your Alexa or smart home devices connected on an isolated VLAN so that they can't "see" and spy on your own internal network
Ability to finely control inter-VLAN routing - this is one we shall touch on later on, as VLANs allow a network administrator to establish rules for how different VLANs can connect to each other and with the internet - for example a "kids" VLAN that doesn't have access to a specific game after 10PM
VLAN-aware Switches and Routers
Before we move around in to additional information about implementation, I want to briefly discuss the hardware side of things. This will be a very high-level overview as there are many resources for learning the specific "behind-the-scenes" of how VLANs work in networking hardware. I am aiming to ensure it is easily understood for a newcomer or beginner.
Remember: Part of setting up VLANs is learning the quirks of one's particular equipment rather than to assume one vendor will undoubtedly be like another in their VLAN implementation. You will see different manufactures will have slightly different implementations, however the overall concept remains exactly the same!
Why can't all routers and switches support VLANs, isn't it only a software thing?
Yes and no. By the end of your day, all a VLAN is really in practice is a little 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 can be done in software or hardware. BUT, understand that our routers and switches are usually not very powerful when it comes to software tasks.
Network switches are low-power, efficient devices that a straightforward job really efficiently with hardware. That is why a Mikrotik CRS-328 can switch 63gbps of traffic when it has a single 800 mhz CPU. When it needs to utilize its CPU to route traffic, like routing traffic in one VLAN to some other, that throughput number falls to under 500 mbps.
There are 3 answers to this problem:
Throw more power at it: a powerful CPU are designed for VLAN tagging without much issue - look at a Proxmox or other virtualization server, that may handle VLAN traffic
Use hardware that is optimized for the task: find a "managed" switch that has Layer 2 capability in the event that you just need simple VLAN capability or Layer 3 Hardware capability if you want the switch to manage to route traffic between different VLANs
Be OK with lower speeds or decreased efficiency. This is sometimes the answer, specifically for the homelab. For example, my travel router, a GL-AR750s has OpenWrt installed and can do VLAN filtering through software. I am fine that this isn't probably the most efficient setup possible and value the convenience and cost effectiveness more in this example. This may apply to 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
Listed below are some important terms and concepts to understand. These will allow you to grasp diagrams and types of VLAN setups and translate them to your personal equipment:
VLAN ID/VID:
This can be the number, 1-4095 of the VLAN. This is used by networking equipment to recognize and group members of exactly the same VLAN together. It is important to realize that the quantity is all that matters. If you label VLAN 10 "Sally" using one switch and VLAN 10 "Jimmy" on another, the only thing the switches really value is the VID
Tag
A VLAN tag is a little piece of information put into a packet that tells networking equipment which VLAN that packet belongs to.
Tagged
When setting up a VLAN-aware switch, you'll often run into the word tagged or untagged by means of a checkbox or dropdown menu for each physical port on the switch or router (and sometimes the "CPU" is known as its own port, like regarding OpenWrt or "bridge" in the case of Mikrotik - this is very important to inter-VLAN routing which will be discussed later). When choosing "tagged" - you are indicating to the switch that the traffic on that port with the VID you are 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 grab that traffic and filter it. This is useful 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 since it leaves the switch, so that it will show to the connected device as if 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, and therefore packets coming into/leaving the switch without tag will be regarded as part of this VLAN. You might be confused by the point of the when you also have untagged/access ports. On Mikrotik in the event that 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
vlans
This is one of the best and what I think is the most powerful elements of VLANs, trunks. A trunk is formed when you tag multiple VIDs about the same port using one switch. This port may then be used for connecting another VLAN-aware switch and be able to utilize 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 also have the excess 10G SFP+ ports on the next switch, while still keeping exactly the same network segmentation.