通讯时借用Ethernet0口的IP地址，这样设置有节约IP地址资源等好处,
源码:--------------------------------------------------------------------------------
E0 S0 S0 E0
-----------------RouteA-----------RouteB----------------
172.16.1.1/24 172.16.2.1/24
--------------------------------------------------------------------------------
RouteA
interface serial0
ip unnumbered ethernet0
interface ethernet0
ip address 172.16.1.1 255.255.255.0
RouteB
interface serial0
ip unnumbered ethernet0
interface ethernet0
ip address 172.16.2.1 255.255.255.0
那么两个s端口的地址分别是 172.16.1.1/24 和 172.16.2.1/24
