Many users including me [I use kubuntu in pen drive] have faced the problem with it’s default Network Manager! It just doesn’t seem to remember my Network settings which is very annoying indeed! So here is how i sorted it out, see if this works for you as well:

sudo apt-get remove network-manager-kde [or knetwork-manager or network-manager]

Now with your favourite text editor [I’m using kwrite] open up:

sudo kwrite /etc/network/interfaces

auto eth0

iface eth0 inet dhcp [incase of static just put “static” instead of dhcp]

address [your IP address]
netmask [your netmask]
gateway [your gateway address]

^….Not with [ brackets! ]

Now create if you don’t have already a file in /etc/resolv.conf and add your DNS servers:

namserver [your DNS]

For multiple nameservers just keep on adding “nameserver address”  in a new line.

Then do sudo /etc/init.d/networking restart

See if this works!