A virtual LAN (VLAN) is any broadcast domain that is partitioned and isolated in a computer network at the data link layer (OSI layer 2).LAN is an abbreviation of local area network.
To subdivide a network into virtual LANs, one configures a network switch or router. Simpler network devices can only partition per physical port (if at all), in which case each VLAN is connected with a dedicated network cable (and VLAN connectivity is limited by the number of hardware ports available). More sophisticated devices can mark packets through tagging, so that a single interconnect (trunk) may be used to transport data for multiple VLANs. Since VLANs share bandwidth, a VLAN trunk can use link aggregation and/or quality-of-service prioritization to route data efficiently.
VLANs allow network administrators to group hosts together even if the hosts are not on the same network switch. This can greatly simplify network design and deployment, because VLAN membership can be configured through software. Without VLANs, grouping hosts according to their resource needs necessitates the labor of relocating nodes or rewiring data links.
A basic switch not configured for VLANs has VLAN functionality disabled or permanently enabled with a default VLAN that contains all ports on the device as members. Every device connected to one of its ports can send packets to any of the others. Separating ports by VLAN groups separates their traffic very much like connecting the devices to another, distinct switch of their own.
Configuration of the first custom VLAN port group usually involves removing ports from the default VLAN, such that the first custom group of VLAN ports is actually the second VLAN on the device, in addition to the default VLAN. The default VLAN typically has an ID of 1.
If a VLAN port group were to exist only on one device, no ports that are members of the VLAN group would need to be tagged. These ports would hence be considered “untagged”. It is only when the VLAN port group is to extend to another device that tagging is used. Since communications between ports on two different switches travel via the uplink ports of each switch involved, every VLAN containing such ports must also contain the uplink port of each switch involved, and these ports must be tagged. This also applies to the default VLAN.
Some switches either allow or require that a name be created for the VLAN, but only the VLAN group number is important from one switch to the next.
Where a VLAN group is to simply pass through an intermediate switch via two pass-through ports, only the two ports must be a member of the VLAN, and are tagged to pass both the required VLAN and the default VLAN on the intermediate switch.
Management of the switch requires that the administrative functions be associated with one of the configured VLANs. If the default VLAN were deleted or renumbered without first moving the management connection to a different VLAN, it is possible for the administrator to be locked out of the switch configuration, requiring a forced clearing of the device configuration (possibly to the factory default) to regain access or physical access to the switch if it has a console port or other means of direct management.
Switches typically have no built-in method to indicate VLAN port members to someone working in a wiring closet. It is necessary for a technician to either have administrative access to the device to view its configuration, or for VLAN port assignment charts or diagrams to be kept next to the switches in each wiring closet. These charts must be manually updated by the technical staff whenever port membership changes are made to the VLANs.
Remote configuration of VLANs involves the risk for the administrator to cut off communications accidentally and lose connectivity to the devices they are attempting to configure. Actions such as subdividing the default VLAN by moving the switch uplink ports into a separate new VLAN can suddenly terminate all remote connectivity, requiring the device to be physically accessed at the distant location to continue the configuration process.
Generally, VLANs within the same organization will be assigned different non-overlapping network addresses. This is not a requirement of VLANs. There is no issue with separate VLANs using identical overlapping address ranges (e.g. two VLANs each use the private network 192.168.0.0 / CIDR 16). However, it is generally not possible to route data between two networks with overlapping addresses, so if the goal of VLANs is segmentation of a larger overall organizational network, non-overlapping addresses must be used in each separate VLAN.