Networking Explainer

VLANs: One Switch, Many Networks

One switch, three departments, and none of them can see each other

Lesson notes

Key points

Picture one switch in a faculty building shared by Mining, Geomatics and Admin, all flooding broadcasts to each other.
A VLAN, or virtual LAN, splits that one switch into separate logical networks, each its own broadcast domain.
Access ports carry one VLAN, while trunk ports carry many by adding an 802.1Q tag holding the VLAN ID.
Guess this: can VLAN 10 talk to VLAN 20 through the switch alone? No, traffic between VLANs needs a router or Layer 3 switch.

Definitions

Broadcast domain: The set of devices that receive a frame sent to the broadcast address FF:FF:FF:FF:FF:FF.

Takeaway: A VLAN is a separate broadcast domain; crossing VLANs needs routing.

Quick check: What device is needed for hosts in different VLANs to communicate?

A hub between ports
A router or Layer 3 switch
A longer trunk cable

Each VLAN is a separate network, so traffic between them must be routed at Layer 3.

Broadcast domainVLAN802.1Q trunkInter-VLAN routing

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