partial network sniffer

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I'm attempting to learn how to put together a simple network sniffer which will do the following: 1) detect who is on the network (what users are on) 2) detect what equipment is on the network: such as: routiner(s) printer(s) any cell phone connected and other equipment. 3) and return any info about each of them. 4) and be updated all the time. This would not be a full fledged sniffer.