What are VLANs?

If you’ve been in the IT field, or trying to learn more about networking, VLANs were one of those topics that sounded way more complicated than they actually were. Once I finally saw them in action, though, they became much easier to understand. At their core, VLANs — or Virtual Local Area Networks — are simply a way to logically separate devices on the same physical network. Instead of every device talking freely across one giant network, VLANs let us organize and segment traffic into smaller, cleaner sections. For example, VLANs for Guest, IoT, production traffic, management interfaces, etc.

A good way to explain VLANs is by thinking about an office building. Even though everyone works in the same building, departments are usually separated into different rooms or floors. Accounting probably doesn’t need direct access to engineering systems, and guest visitors definitely shouldn’t be sitting on internal company resources floors. VLANs work the same way on a network. Devices can share the same switches and cabling infrastructure, but still remain isolated from each other logically.

One of the biggest reasons VLANs are used is to segment traffic. Traffic segmentation helps reduce unnecessary broadcasts, improves security, and makes troubleshooting much easier. For example, an employee workstations may be placed in one VLAN, servers in another VLAN, voice phones in another, and guest Wi-Fi users in a completely separate VLAN. Even though everything may plug into the same switch stack, the traffic stays separated unless a router or Layer 3 switch is specifically configured to allow communication between them.

This becomes especially important in larger environments. Imagine having hundreds or thousands of devices all sitting in one flat network. Broadcast traffic would start flooding everywhere, troubleshooting would become painful, and security risks would increase significantly. VLANs help contain that chaos. By keeping devices grouped by purpose, department, or security requirements, networks become much more scalable and easier to manage. Additionally, when you have a bunch of traffic hitting the switch all at once, it typically handles everything in a first packet in, first packet out scenario. In bigger environments, that may start to degrade VoIP call performance. If all of your phones are on one VLAN, you can configure QOS (Quality Of Service) settings so that packets pertaining to the voice VLAN are always prioritized so calls don’t cut out or become static.

Another thing to know is that VLANs are everywhere. Even small businesses use them whether they realize it or not. If a company has separate Wi-Fi networks for employees and guests, there’s a very good chance VLANs are involved behind the scenes. In enterprise environments, VLANs become even more critical because they help support things like IP phones, wireless access points, security cameras, printers, and data center traffic — all while keeping traffic organized and controlled. Even home users use VLANs without knowing if they have a guest network setup. For example, if you go onto our router and enable the guest network and select to isolate the devices, that very likely uses VLANs with some firewall rules to only allow outbound traffic.

When you start working with managed switches, you’ll also hear terms like “access ports” and “trunk ports.” An access port typically carries traffic for a single VLAN and connects to end devices like PCs or printers. A trunk port, on the other hand, carries multiple VLANs between switches, routers, or wireless infrastructure. Understanding that difference is one of those foundational networking concepts that pays off everywhere later on, especially when you begin diving into switching, wireless, and network design.

With VLANs, you don’t necessarily need separate physical switches for every department or type of device. VLANs let engineers build flexible and efficient networks using the same infrastructure while still maintaining separation and control. It’s one of those technologies that quietly powers almost every modern network, even if most users never realize it exists.

If you’re just breaking into networking, don’t stress if VLANs seem confusing at first. Almost everyone struggles with the concept initially because it mixes logical networking with physical infrastructure. Once you lab it up and actually see devices separated into different VLANs, it starts making a lot more sense. VLANs are one of the foundational building blocks of networking, and understanding them early will make so many other topics easier down the road.

How do I configure VLANs?

Now that you have a brief overview on what VLANs are, let’s provide a quick little demo, in-depth, on how they work. The following commands can be used on Cisco switches to configure VLANs. There are a couple things to note:

1. VLANs ID are a unique number given to the VLAN. If you want a VLAN to span multiple switches over a cable connected, you need to make sure that VLAN is on the uplink, or downlink port of the switch, and that the VLAN ID is configured on the switch.
2. VLAN names, are unique to the switches. You can make VLAN 10 on switch 1 as “Employee Network” and VLAN 10 on switch 2 as “Employee WiFi”. Even though they are two separate names, as long as the VLAN ID is the same on both, and is tagged on all ports, it will work the same.
3. There are a little over 4,000 usable VLANs. If you run out (you won’t in a lab, but many big businesses do) you can separate sites with Layer 3 routing, and reuse VLANs at the various sites.

How to configure a VLAN on a switch:

                            Switch(config)# vlan 10
                            Switch(config-vlan)# name VOIP_10.0.1.0/24
                            
                        

In this example, I created VLAN 10 with the name of VOIP_10.0.1.0/24. You can name this whatever you want, but I find it easier to also define the subnet in the name, as it's easier for troubleshooting.

How to configure an access port with a VLAN:

                            Switch(config)# int Te1/0/37
                            Switch(config-if)# switchport mode access
                            Switch(config-if)# switchport access vlan 10
                            
                        

In this example, I configured a switchport to have an access vlan of 10. This means, all devices on that port will get a IP on VLAN 10, whether they're tagging traffic or not (Telling the switch what VLAN to use).

How to configure a trunk port as a VLAN:

                            Switch(config)# int Te1/0/37
                            Switch(config-if)# switchport mode trunk
                            Switch(config-if)# switchport trunk native vlan 10
                            Switch(config-if)# switchport trunk allowed vlan all
                            
                        

With this, I configured the native VLAN on this port to be on 10, but allowed all other VLANs. This means, if the device doesn't send any traffic with a VLAN ID, it will drop it onto VLAN 10, but if the device sends packets with another VLAN ID in the packet, it will use that VLAN instead.