Here is how I explain VLANS to my new techs. VLANs are like social clubs for Ethernet Packets. Each ethernet packet gets a Tag which defines its club affiliation. Then, packets with the same tag group together and those with different tags do not acknowledge each others existence. Those with no tags are simply allowed to stay in the default and everyone in a club ignores them as well. The only way to get into a club if you did not start out in one is to go thru a routing device. So, VLAN 105 club only sees packets in 105 club and VLAN 99 only sees packets in 99 Club and they both ignore the packets in the default who are not in any club. Even though they are all on the same wire, they will not mix. Believe it or not, I have gotten farther with that explanation over the years than any other I have tried....
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