Understanding VLAN Segmentation and Switching Behavior [Interactive Simulation]
Mohamad's interest is in Programming (Mobile, Web, Database and Machine Learning). He is studying at the Center For Artificial Intelligence Technology (CAIT), Universiti Kebangsaan Malaysia (UKM).
Virtual Local Area Networks (VLANs) are a foundational concept in modern Ethernet networks, yet they are often misunderstood because their effects are largely invisible. Frames are forwarded, flooded, or dropped silently by switches, and learners are frequently expected to understand this behavior without ever seeing it. This article explains how VLANs influence switching behavior at Layer 2 and uses interactive simulations to make those decisions observable. By working through the simulations, readers gain a clear understanding of what VLANs do, what they intentionally prevent, and why routing is required when multiple VLANs are involved.
A VLAN is a logical segmentation mechanism implemented on switches that creates separate broadcast domains within the same physical infrastructure. Even though devices may be plugged into the same switch chassis, VLANs cause those devices to behave as if they are connected to different switches. VLANs operate entirely at the Data Link layer and do not rely on IP addressing. Their primary purpose is to control how Ethernet frames are forwarded and to limit the scope of broadcast traffic.
In a traditional flat switched network with no VLANs, all hosts belong to a single broadcast domain. Any broadcast frame, such as an ARP request or a DHCP Discover message, is flooded to every device connected to the switch. As the network grows, this broadcast traffic can consume significant bandwidth and processing resources, and a single malfunctioning device can negatively affect the entire network. VLANs solve this problem by dividing a large broadcast domain into multiple smaller ones, each isolated from the others.
VLANs also change how switches learn and store MAC address information. In a VLAN-aware switch, the MAC address table is no longer a simple mapping of MAC addresses to ports. Instead, each entry is associated with a VLAN. The switch learns MAC addresses separately for each VLAN and makes forwarding decisions based on both the destination MAC address and the VLAN ID. This ensures that traffic remains confined to the correct logical network.
When unicast traffic is sent between two hosts in the same VLAN, normal switching behavior applies. The switch learns the source MAC address, looks up the destination MAC address in the VLAN-specific MAC table, and forwards the frame to the correct port if the destination is known. If the destination MAC address has not yet been learned, the switch performs unknown unicast flooding, but only within that VLAN. Hosts in other VLANs never receive the frame.
Broadcast traffic is treated similarly. Broadcast frames are always flooded, but VLANs strictly limit where that flooding occurs. A broadcast sent by a host in VLAN 10 is forwarded only to ports that belong to VLAN 10. Devices in VLAN 20 never see the frame. This broadcast containment is one of the most important benefits of VLANs, as it limits the impact of broadcast storms and improves overall network stability.
The first interactive project, titled “VLAN Segmentation + Broadcast Containment Simulator,” focuses exclusively on Layer 2 behavior. In this simulation, users assign each host to either VLAN 10 or VLAN 20 and observe how the switch forwards frames. Unicast traffic between hosts in the same VLAN behaves normally, while unicast traffic destined for a host in a different VLAN is dropped. Broadcast traffic is flooded only within the originating VLAN. The event log explains each forwarding decision, including when frames are discarded due to VLAN mismatches. This simulator intentionally excludes routing to reinforce the idea that VLANs alone do not enable communication between separate logical networks.
As users interact with the simulator, a natural question arises: how do hosts in different VLANs ever communicate in real networks? The answer is that VLANs do not provide inter-VLAN communication on their own. Communication between VLANs requires a Layer 3 device, such as a router or a Layer 3 switch, to perform routing. This distinction is critical and often misunderstood by beginners.
The second project, “VLAN Simulator with Optional Inter-VLAN Routing,” builds directly on the first by introducing a simple Layer 3 component. In this simulation, each VLAN is associated with a default gateway. When traffic is sent between VLANs, the switch forwards the frame to the appropriate gateway, and the router performs Layer 3 routing before delivering the traffic to the destination VLAN. The event log explicitly states when traffic is forwarded to the default gateway and when routing occurs between VLANs. This makes the separation between switching and routing clear and prevents the common misconception that VLANs themselves perform routing.
By separating the learning experience into two distinct projects, these simulations preserve a clean mental model. The first project teaches pure Layer 2 behavior and VLAN isolation, while the second introduces routing as an additional capability rather than an inherent feature of VLANs. This progression mirrors real-world network design and aligns closely with how VLANs and inter-VLAN routing are tested in certification exams.
Ultimately, VLANs are not merely configuration commands or colored diagrams on a switch. They are logical policy boundaries enforced by switch logic. Through interactive simulation, learners can see how frames move, why broadcasts are contained, and why routing is required to connect separate VLANs. This visibility transforms VLANs from an abstract concept into an understandable and practical networking tool.