Why nmap -PU can't find any host available,even the target machine return an ICMP port unreachable p

Why nmap -PU can't find any host available,even the target machine return an ICMP port unreachable packet.


I run [nmap -sn -n -PU xxx.xxx.xxx.xxx/24]for host discovery，The description of -PU in official Documents:


[Upon hitting a closed port on the target machine, the UDP probe should elicit an ICMP port unreachable packet in return. This signifies to Nmap that the machine is up and available.]


Meaning that when the target host returns ICMP unreachable [Type3 Code3].Nmap will report this host is available.
I use wireshark to capture.And wireshark displays many hosts replied ICMP port unreachable,but the final result is shown by nmap:"Nmap done: 256 IP addresses (0 hosts up)"，Meaning is no host is available. Selinux and iptables are disable.


My computer:
Centos 6.5

Nmap 6.40

下面是在windows XP下运行的截图，在centos下运行一样，因为截图不方便所以在window下上图。