在局域网中扫描一台机器是否在线，或者某个端口在目标机器上是否开启，
可以用这个命令方便的进行查询。
在局域网中DHCP开启的情况下，这个命令尤其有用，设想知道目标机器的MAC地址，
扫描得到结果后，即可以知道该机器的IP地址，不必反复的跑离座位或者到服务器上去查找目标机器的IP地址。
nmap -v -sU 10.9.8.0/24 -p 1137
结果如下：
Host 10.9.8.217 appears to be up ... good.
Interesting ports on 10.9.8.217:
PORT STATE SERVICE
1137/tcp open unknown
MAC Address: 00:0E:09:AC:73:31 (Shenzhen XXX Software Co.)
Host 10.9.8.42 appears to be up ... good.
Interesting ports on 10.9.8.42:
PORT STATE SERVICE
1137/udp closed unknown
MAC Address: 00:0C:29:60:4A:A7 (VMware)
Host 10.9.8.71 appears to be up ... good.
Interesting ports on 10.9.8.71:
PORT STATE SERVICE
1137/udp closed unknown
MAC Address: 00:16:EC:B0:8A:33 (Elitegroup Computer Systems Co.)