我打开路由器，如何知道外网IP

用tracert命令啊，例如tracert www.baidu.com内网IP地址的下一个就是公网IP地址。
C:\Documents and Settings\user>tracert www.baidu.com

Tracing route to www.a.shifen.com [119.75.213.61]
over a maximum of 30 hops:

1 1 ms <1 ms <1 ms 192.168.100.100
2 11 ms 19 ms 9 ms 123.65.216.1
3 5 ms 3 ms 5 ms 61.235.96.133
4 * * * Request timed out.
5 5 ms 5 ms 6 ms 61.237.112.73
6 41 ms 32 ms 33 ms 61.237.112.145
7 31 ms 32 ms 32 ms 61.237.3.6
8 35 ms 39 ms 33 ms 222.35.251.138
9 33 ms 33 ms 34 ms 222.35.251.22
10 34 ms 33 ms 35 ms 222.35.251.202
11 33 ms 34