用nmap查找现场主机

我试图用nmap在networking上find现场主机：

nmap -sP 192.168.3.0/24 Starting Nmap 5.21 ( http://nmap.org ) at 2012-04-10 10:28 EEST Nmap scan report for km-localhost (192.168.3.1) Host is up. Nmap scan report for km-localhost (192.168.3.6) Host is up (0.00067s latency). MAC Address: 00:26:18:B8:4E:B8 (Asustek Computer) Nmap scan report for 192.168.3.7 Host is up (0.00016s latency). MAC Address: 00:0E:2E:2B:E7:BD (Edimax Technology Co.) Nmap scan report for km-localhost (192.168.3.11) Host is up (-0.10s latency). MAC Address: 6C:F0:49:74:3A:A2 (Giga-byte Technology Co.) Nmap scan report for 192.168.3.15 Host is up (0.00057s latency). MAC Address: 00:1F:C6:CF:76:48 (Asustek Computer) Nmap scan report for km-localhost (192.168.3.22) Host is up (0.0030s latency). MAC Address: 00:12:17:6B:0C:DF (Cisco-Linksys) Nmap scan report for 192.168.3.24 Host is up (-0.10s latency). MAC Address: 00:02:B3:65:2D:1B (Intel) Nmap scan report for km-localhost (192.168.3.25) Host is up (0.00014s latency). MAC Address: 00:C0:26:A7:6B:0F (Lans Technology CO.) Nmap done: 256 IP addresses (8 hosts up) scanned in 4.08 seconds 

所以nmap发现了8个主机。 现在问题来了，当我试图使用IP列表而不是cidr。

  nmap -sP 192.168.3.1 192.168.3.6 192.168.3.7 192.168.3.11 192.168.3.15 192.168.3.22 192.168.3.24 192.168.3.25 Starting Nmap 5.21 ( http://nmap.org ) at 2012-04-10 10:33 EEST Nmap scan report for km-localhost (192.168.3.1) Host is up. Nmap scan report for km-localhost (192.168.3.15) Host is up (-0.10s latency). MAC Address: 00:1F:C6:CF:76:48 (Asustek Computer) Nmap done: 8 IP addresses (2 hosts up) scanned in 0.24 seconds 

在这里，我提供了所有活动的IP地址列表，就像您在前面的命令中看到的一样，但8个中只有2个显示为活动状态。 任何人都可以解释这个nmap的行为，也许告诉工作？

我想在shell脚本中使用nmap来快速确定活动主机。 以前我使用'fping -a'命令，但是nmap似乎在发现防火墙后面的主机上更好，所以我想切换到它而不用修改我的脚本太多。 任何帮助将不胜感激。

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