Ethernet: VLANs

In an Ethernet LAN a group of devices that receive a broadcast sent by any other device in the same group is called a broadcast domain.  A VLAN is simply a defined subset of switch ports designated to be in the same broadcast domain.

Ports can be grouped into different VLANs on a single switch, and on multiple connected switches as well.  Multiple VLANs create multiple yet contained broadcast domains.  Any broadcast sent by one device on a VLAN is forwarded to all other devices on the same VLAN and is not forwarded to devices on other VLANs.

Best practices dictate a one to one relationship between VLANs and IP subnets.  While it is possible to configure multiple IP Subnets on a single VLAN it is considered bad design to do so.

To forward frames between two devices in different VLANs a Multilayer Switch (MLS) or router is needed.



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