Which tagging method supports thousands of VLANs?

The tagging method that supports thousands of VLANs is 802.1Q. This standard is specifically designed for VLAN (Virtual Local Area Network) tagging, allowing multiple VLANs to coexist on a single physical network bridge or switch.

802.1Q introduces a mechanism that adds a tag to Ethernet frames, enabling the identification of the VLAN to which each frame belongs. The tag comprises a 12-bit VLAN ID, which allows for the support of up to 4096 different VLANs. This extensive range is particularly beneficial in complex networks where segmentation and organization of traffic into numerous logical networks are necessary.

In contrast, the other options do not pertain directly to VLAN tagging in the same manner. 802.1X is primarily related to network access control and authentication, 802.1D pertains to the Spanning Tree Protocol which addresses network loops, and 802.3ad is concerned with link aggregation to combine multiple network connections for increased throughput and redundancy. Each of these standards serves its specific purpose and does not provide the VLAN tagging capabilities that 802.1Q does.