VLANs are like classrooms, where the size of the room is relative to the IP address block size. Anytime you try to send a message, like passing a paper note, you have to send it to everyone in the room before your message leaves the room, where the door is the default gateway and the closest person to it hands it off. People in other rooms (VLANs) will not see the note unless it was allowed and explicitly sent to them, in which case the hall monitor (router) will give it to that room. The note is then passed around in that room until it reaches the intended recipient.
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