打开终端，使用管理员权限运行：
nmap -A -T4
等一会儿（一个存活IP大概要扫七八十秒，但是值得）
成功的话会出来：

Starting Nmap 5.00 ( http://nmap.org ) at 2010-05-23 13:24 CST
Interesting ports on 192.168.1.101:
Not shown: 993 filtered ports
PORT      STATE  SERVICE     VERSION
135/tcp   open   msrpc       Microsoft Windows RPC
139/tcp   open   netbios-ssn
445/tcp   open   netbios-ssn
554/tcp   closed rtsp
2869/tcp  open   http        Microsoft HTTPAPI httpd 2.0 (SSDP/UPnP)
5357/tcp  open   http        Microsoft HTTPAPI httpd 2.0 (SSDP/UPnP)
|_ html-title: Service Unavailable
10243/tcp open   http        Microsoft HTTPAPI httpd 2.0 (SSDP/UPnP)
|_ html-title: Not Found
MAC Address: 00:1E:65:ED:73:FA (Intel Corporate)
Device type: general purpose
Running: Microsoft Windows Vista|2008
OS details: Microsoft Windows Vista SP0 or SP1 or Server 2008 SP1
Network Distance: 1 hop
Service Info: OS: Windows

Host script results:
|_ nbstat: NetBIOS name: MOON-PC, NetBIOS user: , NetBIOS MAC: 00:1e:65:ed:73:fa
|  smb-os-discovery: Windows 7 Ultimate 7600
|  LAN Manager: Windows 7 Ultimate 6.1
|  Name: WORKGROUP\MOON-PC
|_ System time: 2010-05-23 13:25:06 UTC+8

OS and Service detection performed. Please report any incorrect results at http://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 78.63 seconds

红字是IP和MAC，紫字是主机名，蓝字是操作系统。怎么样，是否出冷汗了呢？
可见如果没有防火墙，很快别人想知道的都会知道！

推荐装个防护软件，随便都可，连金山网镖乃至360安全卫士都可以，紫字和蓝字将成为如下：

Warning: OSScan results may be unreliable because we could not find at least 1 open and 1 closed port
Device type: general purpose
Running (JUST GUESSING) : Microsoft Windows XP|2003 (89%), Apple Mac OS X 10.4.X (85%)
Aggressive OS guesses: Microsoft Windows XP SP2 (89%), Microsoft Windows XP SP2 or SP3 (87%), Microsoft Windows XP SP3 (86%), Microsoft Windows Server 2003 SP2 (86%), Apple Mac OS X 10.4.9 (Tiger) (PowerPC) (85%)
No exact OS matches for host (test conditions non-ideal).
虽然IP和MAC还可知，但主机名已消失，操作系统也变成了猜测。虽然猜的八九不离十，但也多出个Mac OS。