tcpdump和nmap的问题

tzredhat

请问tcpdump是不是要安装在网关服务器上？如果安装在内网的任意一台机器上，是不是就只能抓到本机的数据包？我试着把网卡设成混杂模式也抓不到其它机器的数据。
另外，本人的机器在192.168.1网段，用nmap扫描本网段的机器很快，但是扫其它网段的机器，就反复出现“RTTVAR has grown to over 2.3 seconds, decreasing to 2.0
RTTVAR has grown to over 2.3 seconds, decreasing to 2.0
RTTVAR has grown to over 2.3 seconds, decreasing to 2.0
RTTVAR has grown to over 2.3 seconds, decreasing to 2.0”，当然最后也能扫出结果，但是时间要长得多。