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默认路由：
（Default route），是对IP数据包中的目的地址找不到存在的其他路由时，路由器所选择的路由。目的地不在路由器的路由表里的所有数据包都会使用默认路由。这条路由一般会连去另一个路由器，而这个路由器也同样处理数据包: 如果知道应该怎么路由这个数据包，则数据包会被转发到已知的路由；否则，数据包会被转发到默认路由，从而到达另一个路由器。每次转发，路由都增加了一跳的距离
默认路由只能存在末梢网络中
Router#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#int s2/0
Router(config-if)#ip route 0.0.0.0 255.255.255.255 13.13.13.100
Router(config)#^Z
Router#
%SYS-5-CONFIG_I: Configured from console by consoleRouter#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static routeGateway of last resort is 13.13.13.100 to network 0.0.0.0
* 0.0.0.0/32 is subnetted, 1 subnets
S* 0.0.0.0 [1/0] via 13.13.13.100
S 8.0.0.0/8 [1/0] via 13.13.13.100
C 10.0.0.0/8 is directly connected, FastEthernet0/0.1
C 11.0.0.0/8 is directly connected, FastEthernet0/0.2
C 12.0.0.0/8 is directly connected, FastEthernet1/0
C 13.0.0.0/8 is directly connected, Serial2/0
Router#