VLAN (IEEE 802.1Q)
Updated
A VLAN is a logical network carried on a shared Ethernet link, identified by a 12-bit tag in each frame as defined by IEEE 802.1Q. One physical link can thus carry up to 4,094 separate networks.
In practice
Cloud interconnections and datacenter cross-connects use VLANs to separate customers and environments. When you order private delivery, a VLAN number is agreed for each connection.