迷惑了我2天的问题，现在终于解决了，答案是网上找到的：

What is a tagged VLAN [Virtual Local Area Network]?
The IEEE 802.1Q standard allows defining an ethernet interface for multiple logical locations (as opposed to specific physical locations). One port can belong to several 802.1Q tagged VLANs. Thus, many host servers may be connected to the same ethernet LAN switch without being in the same collision or broadcast domain inherent in ethernet technology.

What is an untagged VLAN?
Untagged VLANs are the way we physically separate host servers into different broadcast and collision domains. A port (assigned to each PC connected to the LAN) can only belong to one untagged VLAN. Think of these LANs as the physical LAN connections within the logical or virtual LAN.

url是：http://www.lockergnome.com/it/2006/11/08/vlan-tagging/

也许是我笨吧，昨晚一直没搞通这两种vlan的区别，现在看了上面的两句话，一下子就明白了，很高兴啊很高兴！