I had a wired situation tomorrwo, could get the ip address, set up apt.conf and proxy correctly, but could not connect network, could ping a inner desttop, then my friend found that it responses twice when ping this ip, so it probably is that the ip is used by other people stactly, so I need to get a new automatic ip address, here is what I did,

 

1. sudo dhclient -r or sudo dhclient -r eth0

r for release.

2 sudo dhclient or sudo dhclient eth0

obtain fresh IP

3 sudo dhclient -v -r eth0 or sudo dhclient -v eth0

v for verbose, will show the detail enformation.  

 

other option may be useful

ifdown eth0

ifup eth0

/etc/init.d/networking restart