Theme: Organization of work of local networks. Networking by domain structure. Contents

Virtual LANs (VLANs) and Trunking

As stated earlier, physical LANs include all devices on a hub, on a switch or on a group of interconnected switches.  When using physical LANs exclusively, you would need at least as many switches as you have LANs that you want to implement.  Virtual LAN or "VLAN" technology enables an organization to use one physical switch to serve multiple VLANs, in many cases saving hardware costs by enabling the organization to buy one large switch instead of many small ones.

The way VLANs are identified is simple:  each switch port that will be used to serve one VLAN is configured to have the same VLAN number assigned.  So if I have a switch with twenty ports, I might configure the switch ports so that six switch ports are assigned to VLAN "1", five others to VLAN "2" and nine switch ports to VLAN "3".  The functioning of VLANs and LANs are identical as far as the computers connected to the switch are concerned.

VLANs can span across multiple interconnected switches through a function called "trunking".  With trunking configured, all physical ports on the multiple switches that have the same VLAN number are considered one VLAN.