A VLAN (Virtual LAN) is an attribute that can be applied to network packets. Network packets can be "tagged" into a numbered VLAN. A VLAN is a security feature used to completely isolate network traffic at the switch level. VLANs are completely separate and mutually exclusive. The Red Hat Virtualization Manager is VLAN aware and able to tag and redirect VLAN traffic, however VLAN implementation requires a switch that supports VLANs.
At the switch level, ports are assigned a VLAN designation. A switch applies a VLAN tag to traffic originating from a particular port, marking the traffic as part of a VLAN, and ensures that responses carry the same VLAN tag. A VLAN can extend across multiple switches. VLAN tagged network traffic on a switch is completely undetectable except by machines connected to a port designated with the correct VLAN. A given port can be tagged into multiple VLANs, which allows traffic from multiple VLANs to be sent to a single port, to be deciphered using software on the machine that receives the traffic.