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