两DHCP server，1号server地址分配范围为172.16.0.0/16，2号为192.168.74.0/24。
只有1号工作时，1号的log如下：
Aug 3 17:44:31 CentOS_Server dhcpd: DHCPDISCOVER from 00:50:56:c0:00:06 (winxp-
client) via eth0
Aug 3 17:44:32 CentOS_Server dhcpd: DHCPOFFER on 172.16.0.128 to 00:50:56:c0:00
:06 (winxp-client) via eth0
Aug 3 17:44:32 CentOS_Server dhcpd: DHCPREQUEST for 172.16.0.128 (172.16.0.2) f
rom 00:50:56:c0:00:06 (winxp-client) via eth0
Aug 3 17:44:32 CentOS_Server dhcpd: DHCPACK on 172.16.0.128 to 00:50:56:c0:00:0
6 (winxp-client) via eth0
注意上面dhcp工作的四个步骤，很详细了吧？此时winxp-client获得了地址172.16.0.128。
打开2号server,
在winxp-client上执行ipconfig /release ipconfig/renew
再看1号server的log:
Aug 3 17:41:35 CentOS_Server dhcpd: DHCPDISCOVER from 00:50:56:c0:00:06 via eth
0
#这个DHCPDISCOVER包被两台服务器都收到了
Aug 3 17:41:35 CentOS_Server dhcpd: DHCPREQUEST for 192.168.74.1 (192.168.74.25
4) from 00:50:56:c0:00:06 via eth0: wrong network.
#2号server已经offer了一个地址192.168.74.1，winxp-client希望使用这个地址，所以广播出DHCPREQUEST,也被1号server收到，但1号server不负责192.168.74.0/24,所以是wrong network，发出下面的DHCPNAK包：
Aug 3 17:41:35 CentOS_Server dhcpd: DHCPNAK on 192.168.74.1 to 00:50:56:c0:00:0
6 via eth0
#offer一个本机负责分配的地址，如果winxp-client的DHCPREQUEST被2号server拒绝，可能会用得着哦：
Aug 3 17:41:36 CentOS_Server dhcpd: DHCPOFFER on 172.16.0.128 to 00:50:56:c0:00
:06 (winxp-client) via eth0