In order to correctly spoof an IP address you must not only use the -P0 option but you must also specify what interface you are you using. Like so:


nmap -sS -e eth0 -P0 -S 255.255.255.255 0.0.0.0

or if you are ppp

nmap -sS -e ppp0 -P0 -S 255.255.255.255 0.0.0.0

Hope this helps.