VLANs are a bit like paying pass the parcel. Trunk ports are the same as when the music is playing. You take the parcel (or packet) and pass it on leaving all the wrapping (or tags) alone with all the data from various networks inside the wrapping. When you reach an access port the music stops, and you open the wrapper and see only the traffic for that network. Different access ports in different VLANS will open the parcel to different wrappers and so see traffic from different virtual LANs.
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