使用ifconfig在Android中打开网络，但不起作用

Question

I have tried to use adb shell to operate the network interface of my phone(HTC Desire Z). You can see in the image ,I ping a site first,then turn 3G network down ,and ping it again ,it said Network is unreachable . But when I turn it on ,and use ifconfig and confirm that it is really on .But when I ping it it always said that Net work is unreachable . Who can help me ?!!?

This is my console content :(btw ,I am a new user and not allowed to post image)

ping 211.69.198.222
PING 211.69.198.222 (211.69.198.222) 56(84) bytes of data.
64 bytes from 211.69.198.222: icmp_seq=1 ttl=46 time=356 ms
64 bytes from 211.69.198.222: icmp_seq=2 ttl=46 time=294 ms
64 bytes from 211.69.198.222: icmp_seq=5 ttl=46 time=342 ms
64 bytes from 211.69.198.222: icmp_seq=6 ttl=46 time=341 ms
64 bytes from 211.69.198.222: icmp_seq=8 ttl=46 time=342 ms
64 bytes from 211.69.198.222: icmp_seq=9 ttl=46 time=330 ms
^C
--- 211.69.198.222 ping statistics ---
9 packets transmitted, 6 received, 33% packet loss, time 8025ms
rtt min/avg/max/mdev = 294.831/334.533/356.018/19.263 ms
# ifconfig rmnet0
rmnet0: ip 172.22.39.243 mask 255.255.255.248 flags [up broadcast running multicast]
# ifconfig rmnet0 down
# ping 211.69.198.222
connect: Network is unreachable
# ifconfig rmnet0 up
# ping 211.69.198.222
connect: Network is unreachable
# ping 211.69.198.222
connect: Network is unreachable
# ifconfig rmnet0
rmnet0: ip 172.22.39.243 mask 255.255.255.248 flags [up broadcast running multicast]

Answer 1

I am the Author and I got the answer. When turn down a interface your IP is released . Using "netcfg rmnet0 dhcp " to make a ip avaible first and then you can ping a site as you want .