Using nmap to change a source address. The commands used are:
...to get a list of available interfaces. When an interface is chosen (in this tutorial, eth0 is chosen) use the name of the interface in the next command:
nmap -e eth0 -S 192.168.1.100 192.168.1.109
...which will use the eth0 interface and spoof a source IP of 192.168.1.100, while scanning 192.168.1.109. Because the source address is spoofed, the return traffic from the target host will not be routed back to us. Thus, all ports will appear to be closed.