On an Ubuntu VM I ran: sudo nmap -sP 192.168.0.* This returned: Starting Nmap 5.00 ( http://nmap.org ) at 2010-12-28 22:46 PST Host 192.168.0.0 is up (0.00064s latency). Host 192.168.0.1 is up (0.00078s latency). Host 192.168.0.2 is up (0.00011s latency). . . . Host 192.168.0.254 is up (0.00068s latency). Host 192.168.0.255 is up (0.00066s latency). The problem is I only have 4 live machines on 192.168.0.* so why did nmap report every ip in the subnet has a live host? The ip address of the Ubuntu machine is 192.168.28.131 From this VM I can ping the live systems on my internal subnet 192.168.0.* and get the expected response. And if I ping a machine that doesn't exist I can get no response as expected.