I have problem with internet connection via cable modem, which uses USB port. I checked if modem is recognised from KDE control center, I saw the modem name the USB informations. I don't know the reason why I cannot connect to internet. I know that I don't have DNS problem yet. Because I cannot ping any IP, well I tried other DNS servers IPs also.
Please help me...

thanks...

aren't there anyone...

Well Im having somewhat the same problem, the difference is that mine worked fine (dont know how) until i unconnect the modem. Now im at the same point.
Try loading linux without your modem hooked up once our in kde then plug it in, There is some people for who that worked, but not for me...
Also what Linux do you use and what is your modem, maybe the company have a driver for it..
Also if your are try to use the internet form a ser other than root make sure it has the premission to do so...
Hope it will help.
If i get mine workin ill tell you how i dod it..

try the command

dhcpcd eth1

while as root in command mode...

Ive gotten mine working but it does not seem to be very stable.
Here what i did.:
I reconfigure my network (Again). Make sure u use DHCP and not LoopBack.
Then reboot the system with the modem NOT connected. Once youre in KDE or wich ever one u are using, Connect the modem.
Open a terminal and do su (If you are not login as root)
Do "ifconfig"
If you see something like
eth0 link encap: Ethernet ...
...
...

And maybe:
lo Link ecnap: Local LoopBack ...

You should be good to go
If you dont see eth0 ... then i dont think it will work (At least it does not for me)

Hope it help....

Or you can just get a LAN card and run it through there. That's what I did and mine works fine.