Monitor the user switch

There are 10 users of my switch, and all of them are connected also on my internet, all of them are obtaining an IP automatically, but some them are hidingly connected their another switch, the reason why my internet accessing so slow, i want to know all users of my switch. Please help me to find a free software to download for monitoring all users of my switch. I want to know all users of my switch. Please help me. Your response will be highly appreciated. Thank you very much.