The IEEE 802.1q standard describes a way to create distinct LANs within the same Ethernet network. Each packet being transmitted contains a VLAN ID. VLAN-enabled switches can manage the traffic by sending given VLAN to selected ports, they may also remove the VLAN ID and turn VLAN packets into regular ones. Thus stations belonging to different VLANs are separated although all of them use the same wires. Moreover any reconfiguration is only matter of software - no changes in the cabling are necessary.
Tahoe routers, apart from standard interfaces like eth0 allow creating VLAN interfaces, e.g. eth0.5, eth0.8, etc. This way certain traffic may be directed to a selected VLAN.
