Vlan tagging (aka RFC 802.1q) is a Layer2 protocol enabling more VLAN on a single ethernet connection. It works tagging ethernet frames so that each port accepts only the configured frames.

The vlan id is a 12bit value (from 0 to 4095), and usually network devices use the 4095 value (0xFFF) for the management network.
But the following command gave me an error
# vconfig add eth1 4095

And I discovered that – actually – the last valid VLAN ID is 4094. Here’s a brief discussion on the subject