Virtual LAN, or VLAN, is a new type of LAN-BN (backbone) architecture which is made possible due to new intelligent high speed switches. VLAN is a network in which software is used instead of hardware to assign computers to LAN segments. By using software, computers can be moved from one segment to another without needing to touch physical cables.
A single switch VLAN operates inside one switch – computers are connected into one switch and assigned by software to different VLAN segments. Computers in the same VLAN act as though they are connected to the same physical switch or hub in a subnet. VLAN switches can also create multiple subnets and act as layer-3 switches, or routers, except the subnets are inside the switch instead of between switches. A broadcast message sent by one computer in one VLAN segment is sent only to computers on the same VLAN. The VLAN can be designed to act as though the computers are connected by a hub or by switches. A switched circuit set up is preferable to shared circuit of hubs, but VLAN switches with the capacity to provide switched circuits for hundreds of computers are more expensive.
The pros of VLANs are that they are faster than traditional LAN-BN routed architectures, and there are better opportunities to manage the flow of traffic. A big benefit is that you do not have to assign computers to subnets based on geographic closeness: a multiswitch VLAN has several switches used to build VLANs, and subnets can be created that contain computers in different buildings, so subnets can be created based on who you are rather than where you are.
Another benefit of VLANS is that the traffic on the LAN and BN can be managed very precisely; therefore faster performance can be obtained by allocating resources to manage broadcast traffic. The ability to prioritize traffic is another benefit. The VLAN tag information included in the Ethernet packet defines which VLAN it belongs to and specifies a priority code based on IEEE 802.1q standard. Therefore you can use QoS capabilities in the data link layer and can connect VOIP telephones directly into the VLAN switch and configure the switch to reserve sufficient network capacity so they will always be able to make and receive calls.
Drawbacks of VLANs include greater cost, greater complexity, and the risk of using newer technologies. Therefore VLANs are typically only used for larger networks.
VLANs work by assigning each computer into a VLAN with a VLAN id number which is matched to a traditional IP subnet so each computer also receives a traditional IP address assigned by the VLAN switch which works as a DHCP server. Most VLAN switches can support 255 VLAN simultaneously, so each switch can support up to 255 separate IP subnets.
Computers are assigned to VLAN and IP subnet based on the physical port (jack the cable plugs into) on the switch they are connected to. The network manager uses software to assign the computers to specific VLANs using their physical port numbers, so it is easy to move a computer from one VLAN to another. If a VLAN switch receives a frame destined for another computer on the same subnet and on the same VLAN switch, the switch acts as a traditional layer-2 switch and it forwards the frame unchanged to the correct computer. If the computer sends a message to a computer in the same subnet but a different VLAN switch, the first VLAN switch changes the Ethernet frame by inserting the VLAN id number and priority code into the VLAN tag field and transmits the frame over the trunk to the other switch which removes the VLAN tag information and transmits it to the destination computer.
Sojourn 