Arp packets are an indicator of what machines are active on your network. ArpSpyX will passively sniff your network for ARP packets and display the IP and MAC address of the machine that generated the packet.
ArpSpyX supports two methods of scanning. The first method is a passive mode which will only listen for traffic without sending any packets. The second method is Active and will send out arp who-has requests for every IP address on your subnet. The passive method can be used for stealth analysis or for looking for ARP poisioning attacks while the active method is better for system administrators looking to gather details about their networks.