vlans

VLANs, or Virtual Local Area Networks, are a method of partitioning a physical network into multiple distinct broadcast domains at the data link layer. By isolating broadcast traffic, VLANs reduce unnecessary traffic and improve security, while enabling more flexible network design and administration across a switched infrastructure.

VLANs are implemented on network switches that support 802.1Q tagging. A switch port can be assigned to a single VLAN, known as an access port, carrying untagged frames for that VLAN. Ports that connect to other switches or to routers are configured as trunk ports and carry traffic for multiple VLANs, with frames tagged by their VLAN identifier. The VLAN ID, ranging from 1 to 4094 (with 0 and 4095 reserved in 802.1Q), identifies the destination VLAN.

Common VLAN designs include static or port-based VLANs, where port assignment determines membership, and dynamic VLANs, where membership is determined by policies such as MAC address. VLANs help contain broadcast domains, support logical grouping of users regardless of physical location, and simplify access control and traffic management. They also enable features such as QoS and security policies to be applied per VLAN.

Inter-VLAN routing: To enable communication between VLANs, traffic must be routed by a layer-3 device, such as a router or a multilayer switch. This can be achieved via traditional router-on-a-stick configurations or internal routing on a dedicated L3 switch. DHCP scopes and IP addressing are typically scoped per VLAN.

Security considerations: VLANs are not a complete security boundary; misconfigurations such as VLAN hopping or trunk misalignment can expose traffic. Best practices include using separate management VLANs, restricting trunk ports with allowed VLAN lists, disabling auto negotiation of trunks, and applying appropriate access controls and monitoring. In planning, administrators map users and devices to VLANs based on function, department, or security requirements and document VLAN topology in network diagrams.