Which tagging method supports thousands of VLANs?

The tagging method that supports thousands of VLANs is 802.1Q. This standard is used for Virtual LANs (VLANs) in Ethernet networks. It works by adding a tag to the Ethernet frame, allowing multiple VLANs to be identified on a single physical network infrastructure. This tagging allows for up to 4096 different VLANs, significantly enhancing network segmentation and management.

The other methods mentioned do not support VLAN tagging in the same manner as 802.1Q. For instance, 802.1D is primarily concerned with the Spanning Tree Protocol (STP), which prevents loops in Ethernet networks but does not define VLANs. Meanwhile, 802.1P focuses on traffic prioritization, facilitating Quality of Service (QoS) rather than VLAN identification. Lastly, 802.1X is a network access control protocol used for authenticating devices wishing to connect to a network, not for VLAN tagging. Thus, 802.1Q stands out as the correct choice for supporting a large number of VLANs.