动态NAT配置
ip nat inside source list 1 pool dongsheng
            access-list 1 permit 10.0.0.1 0.0.0.0(与下面的公网IP对应)
            access-list 1 permit 10.0.0.2 0.0.0.0
            ip nat pool dongsheng 192.16.1.1 192.16.1.2 netmask 255.255.255.0
            注:(一段IP地址范围,动态NAT与静态NAT一样,都是一对一的关系,
如果上面内网是3个IP,则下面也需要3个公网IP,如何设置2个出口呢？在2个串口上设置？)
            !
            interface Ethernet0
            ip address 10.1.1.1 255.255.255.0
            ip nat inside
            no shut
            !
            interface Serial0
            ip address 192.16.1.1 255.255.255.0
            ip nat outside
            no shut
            !