A logical group of devices, (not physical), defined by software. VLANs allow network administrators to Resegment their networks without physically rearranging the devices or network connection. A VLAN (Virtual LAN) is a network composed of logical broadcast domain. Configuration VLANs allows network traffic to be separated logically. Network devices on VLAN1 will not be able to communicate (ping) devices on VLAN2. It is possible to have devices on VLAN1 of a switch communication with VLAN1 on another switch through a method called VLAN trunking. See the diagram below:

vlan

This figure gives you the basic idea of VLAN membership. You can see how the floors of the building are separates and that each department is representing by a different color. The switches lie below and the trunk link is represented by the lightning bolt. Trunk links may also be referred to as .1q or “dot one Q.” This is refers to the IEEE standard of 802.1q which defines the method of vlan Trunking.
Types of Membership :
There are different types of memberships associated with VLANs:
     • Static VLAN
     • Dynamic VLAN
Static VLAN are specified by switch port. For example, a 12 port fast Ethernet switch is split for the creation of 2 VLAN. The first 6 ports are fix with VLAN1 and the last 6 ports are fix with VLAN2. If a machine is moved from port 3 to port 11, it will effectively change VLAN.

Dynamic VLANs are specified by MAC address. Assuming the same scenario, a system administrator will enter MAC addresses for all machines connecting to the switch. These addresses will be stored in a memory inside the switch that forms a database of local MAC addresses. Each MAC address can then be associated with a certain VLAN. This way, if a machine is moved, it will retain the original VLAN membership regardless of its port number.
  VLAN Tagging
Moving VLAN data over multiple switches uses a method called VLAN tagging. The act of VLAN tagging simply adds extra information in the packet header of Ethernet frames so routers know how to pass along the data.
   VLAN Enabled Switches
Not all switches support VLAN. Most “managed” switches including Netgear, HP, and others all support VLAN. Remember that because VLAN tagging is a universal standard, different brands of switches can accomplish the same thing. Data centers are large environments should standardize on a specific platform. Cisco has created proprietary protocols to manage VLANs called VLAN Trunking Protocol or VTP which enables Cisco switches to advertise VLAN routes to other VTP enabled switches. This also allows a system administrator to manage all VLAN from a central point and order all switches to update the VLAN information along the entire network. Most organizations using VLAN have figured out it is worth shelling out the extra cash to go with Cisco equipment and get the extra features and functionality.

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