Interconnection · Lesson 03

How can one switch serve separate networks?

A physical switch can carry traffic for several logical networks. VLANs make that separation explicit, so shared infrastructure does not automatically mean shared broadcast domains.

The shared infrastructure problem

One cable does not have to mean one network.

Offices, labs, guest devices, and administration systems may use the same switches while needing different broadcast domains and different access rules.

A VLAN (Virtual Local Area Network) gives Ethernet frames a logical network identity. Switch ports can place devices into a VLAN, while trunk links carry several VLANs between network devices.

Segmentation is a boundary, not a firewall.VLANs separate local broadcast domains. Traffic between VLANs still needs a router or Layer 3 switch, where policy can be applied.

Build the model

Four pieces of segmentation

01

VLAN ID

Labels a logical Layer 2 network, such as VLAN 20 for guests.

02

Access port

Connects an end device to one configured VLAN.

03

Trunk

Carries traffic for multiple VLANs between network devices.

04

Inter-VLAN routing

Moves traffic between VLANs through a Layer 3 decision point.

Mini experiment

Place the traffic

Select a traffic type and see whether it stays inside a VLAN or needs a Layer 3 boundary.

VLANSame VLAN

The guest devices can exchange local frames without crossing a Layer 3 boundary.

A VLAN keeps its local broadcast traffic within that logical segment.

Knowledge check

Test your segmentation model

Answer five questions. The answer order changes each time.

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