VLANs separate devices into logical networks using VLAN IDs and switch port settings. Switches forward frames within the same VLAN and restrict traffic across VLANs by default. To reach other VLANs, you typically add routing at Layer 3 (e.g., a router or L3 switch). Trunk links carry multiple VLANs using tagging, while access ports carry one VLAN. Common issues come from mismatched VLAN IDs, incorrect tagging on trunks, and missing inter-VLAN routing.

How do VLANs work?

Definition and purpose

A VLAN (Virtual Local Area Network) is a way to divide a physical Ethernet network into multiple logical networks. The main idea is that you can group devices by function or trust level—even if they are plugged into the same physical switches—so that traffic is handled according to the VLAN they belong to.

In practice, VLANs reduce unnecessary broadcast and help with organization and isolation. They do not, by themselves, provide all security properties you might expect from a firewall; they mainly control how switching forwards Layer 2 traffic.

A simple model: switches, VLAN IDs, and forwarding

VLAN behavior is implemented primarily on Ethernet switches:

Each VLAN has an identifier (a VLAN ID).
A switch port is configured as either belonging to a specific VLAN (an “access” port conceptually), or it is configured to carry multiple VLANs (a “trunk” conceptually).
Devices connected to an access port are associated with that port’s VLAN.
When a frame arrives at a switch, the switch uses the VLAN context to decide where to forward it.

With this model, a device in VLAN 10 generally only exchanges Layer 2 traffic with other devices in VLAN 10 (within the same switching domain). If you want VLAN 10 devices to talk to VLAN 20 devices, you need Layer 3 connectivity (routing), because VLANs are a Layer 2 segmentation mechanism.

How multiple VLANs travel: tagging and trunks

When switches need to carry traffic for more than one VLAN across the same physical link, they typically use a trunk-style link that can transport multiple VLANs at once.

To keep VLAN traffic separated on the shared link, the switch uses VLAN tagging. Tagging marks frames with the VLAN ID, so the receiving switch can place the traffic into the correct VLAN context.

Key checks for correctness:

The VLAN IDs must match on both ends.
The trunking/tagging behavior must be consistent (what the link carries, and how tags are interpreted).
The VLAN must be allowed on the trunk (not every system allows every VLAN on every trunk by default).

Inter-VLAN communication and the boundary between VLANs and routing

A common point of confusion is expecting VLANs to “connect” to each other automatically. They generally do not. Instead:

VLANs separate Layer 2 forwarding domains.
Routing (Layer 3) provides connectivity between them.

So, if VLANs are isolated at Layer 2, then communication across VLANs depends on a router or an L3-capable device that has interfaces/addresses for each VLAN (or an equivalent Layer 3 mechanism). Without that, devices in different VLANs can’t reach each other using their IP settings.

This is also where you should be careful about assumptions: VLANs can limit who can communicate at Layer 2, but IP-level reachability and filtering rules depend on the Layer 3 design.

Differences, limits, and what can change the outcome

VLANs work at Layer 2, so several limitations are inherent:

They don’t replace firewalls. If you need policy enforcement between groups, you typically rely on Layer 3 controls rather than VLAN separation alone.
Broadcast and some discovery behaviors are constrained to a VLAN, but higher-level behaviors can still expose services if routing and policies allow it.
The exact operational behavior can vary by vendor and configuration style, and not every network uses the same VLAN features (for example, different tagging modes or native/VLAN handling concepts exist). Because the details depend on your gear, treat this as a conceptual baseline and verify against your switch’s documentation.

Practical checks you can run

To confirm that VLANs are working as intended, you can focus on a few observable configuration facts:

Port membership: verify that the physical ports where devices connect are assigned to the expected VLAN.
VLAN on trunk links: confirm that the VLANs you expect are being carried across switch-to-switch links, and that tagging is enabled/compatible on both ends.
Inter-VLAN paths: if devices in different VLANs must communicate, verify that there is Layer 3 routing (and any policy controls) between those VLANs.
Symptom-driven debugging: if a device “can’t reach” resources, the root cause is often one of: wrong VLAN ID, access vs trunk mismatch, missing VLAN allowed on a trunk, or missing/incorrect routing between VLANs.