You asked for the 3500 , the 2950 is done the same way as the 3500 . Ping address , show arp , write down mac , do a show mac-address-table and this will tell you what port the ip is on . If it is trunked down to the 2950 you may have to get the arp table at the router level and then look for the mac address at the 2950 level .