can't connect to the internet

Open up a terminal window (Applications menu, Accessories) and post the answers to

ifconfig

and to list the PCI adapters detected

lspci

and to show the USB devices you have attached.

lsusb

This information will let you confirm whether your netowrk card is supported...
https://wiki.ubuntu.com/HardwareSupport

eth0 Link encap:Ethernet HWaddr 00:08:02:7c:b2:8c
          UP BROADCAST MULTICAST MTU:1500 Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:36
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)
          Interrupt:10 Base address:0x9000

lo Link encap:Local Loopback
          inet addr:127.0.0.1 Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING MTU:16436 Metric:1
          RX packets:1246 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1246 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:62860 (61.3 KB) TX bytes:62860 (61.3 KB)

00:00.0 Host bridge: ATI Technologies Inc RS200/RS200M AGP Bridge [IGP 340M] (rev 02)
00:01.0 PCI bridge: ATI Technologies Inc PCI Bridge [IGP 340M]
00:06.0 Multimedia audio controller: ALi Corporation M5451 PCI AC-Link Controller Audio Device (rev 02)
00:07.0 ISA bridge: ALi Corporation M1533/M1535 PCI to ISA Bridge [Aladdin IV/V/V+]
00:0a.0 CardBus bridge: Texas Instruments PCI4410 PC card Cardbus Controller (rev 02)
00:0a.1 FireWire (IEEE 1394): Texas Instruments PCI4410 FireWire Controller (rev 02)
00:0b.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL-8139/8139C/8139C+ (rev 20)
00:0c.0 Communication controller: Conexant HSF 56k HSFi Modem (rev 01)
00:10.0 IDE interface: ALi Corporation M5229 IDE (rev c4)
00:11.0 Bridge: ALi Corporation M7101 Power Management Controller [PMU]
00:13.0 USB Controller: NEC Corporation USB (rev 41)
00:13.1 USB Controller: NEC Corporation USB (rev 41)
00:13.2 USB Controller: NEC Corporation USB 2.0 (rev 02)
01:05.0 VGA compatible controller: ATI Technologies Inc Radeon IGP 330M/340M/350M

Bus 003 Device 002: ID 04b4:6560 Cypress Semiconductor Corp. CY7C65640 USB-2.0 "TetraHub"
Bus 003 Device 001: ID 0000:0000
Bus 002 Device 001: ID 0000:0000
Bus 001 Device 001: ID 0000:0000

so far i think its supported, or atleast UBuntu recognizes that i have a realtek ethernet controller

Two commands which may shed light on the issue... please run both, and tell us what the output is:

  sudo ethtool eth0
  sudo dhclient eth0

The ethtool command will show things like whether the card is plugged in. The dhclient command will attempt to get an address from DHCP, and print details about what happens.

If it works, it will end with something like:

  bound to 192.168.1.103 -- renewal in 1786 seconds.

If you see that, try to access the network.

Settings for eth0:
 Supported ports: [ TP MII ]
 Supported link modes: 10baseT/Half 10baseT/Full
                         100baseT/Half 100baseT/Full
 Supports auto-negotiation: Yes
 Advertised link modes: 10baseT/Half 10baseT/Full
                         100baseT/Half 100baseT/Full
 Advertised auto-negotiation: Yes
 Speed: 10Mb/s
 Duplex: Half
 Port: MII
 PHYAD: 32
 Transceiver: internal
 Auto-negotiation: on
 Supports Wake-on: d
 Wake-on: d
 Current message level: 0x00000007 (7)
 Link detected: no
tam@Tamster:~$ sudo dhclient eth0
There is already a pid file /var/run/dhclient.pid with pid 6387
killed old client process, removed PID file
Internet Systems Consortium DHCP Client V3.0.6
Copyright 2004-2007 Internet Systems Consortium.
All rights reserved.
For info, please visit http://www.isc.org/sw/dhcp/

Listening on LPF/eth0/00:08:02:7c:b2:8c
Sending on LPF/eth0/00:08:02:7c:b2:8c
Sending on Socket/fallback
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 5
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 13
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 13
No DHCPOFFERS received.
No working leases in persistent database - sleeping.
--------------------------------------------------------
well that last part seems to be the problem. but me being a noob at ubuntu, i have no idea what to do. X-D
btw, thanks for the help so far.

I think your problem is not in your Ubuntu box, I'd say you don't have the DHCP server enabled on your Linksys WRT160N router. Your ISP may give you an IP through the DHCP but that's the IP that is set at your router. You need still to enable the DHCP server on the private side of your router. Or you can set the IP address for your eth0 interface to some static value in /etc/network/interfaces file.

This part of "ethtool"s output may be the problem:

  Link detected: no

That means it doesn't think it's plugged in. However, it could also mean it hasn't checked yet. Could you run "sudo ifconfig eth0 up" and then run "sudo ethtool eth0" again? This will force it to check the cable.

If you still see no link detected, check your physical connection. The cable may be loose, dirty, or broken. Otherwise, if it says it detected a link, the issue could be (as Juraj suggested) a broken or disabled DHCP server.

Let us know what you find, and we'll go from there.

the rj45 works fine, tested it on my other computer. it does say "link detected: no". the DHCP has always been enabled on my router (on by default). i know its not the NIC card on my computer because when i switch OS, it works fine. i even went as far as setting up a static IP w/ my router, still nothing. here's the readout that i got for sudo ethtool eth0:

Settings for eth0:
 Supported ports: [ TP MII ]
 Supported link modes: 10baseT/Half 10baseT/Full
                         100baseT/Half 100baseT/Full
 Supports auto-negotiation: Yes
 Advertised link modes: 10baseT/Half 10baseT/Full
                         100baseT/Half 100baseT/Full
 Advertised auto-negotiation: Yes
 Speed: 10Mb/s
 Duplex: Half
 Port: MII
 PHYAD: 32
 Transceiver: internal
 Auto-negotiation: on
 Supports Wake-on: d
 Wake-on: d
 Current message level: 0x00000007 (7)
 Link detected: no

Funny that it has negotiated down to a half-duplex 10Mb/s link and that the link is not detected.
Your router handles 100Mb/s full-duplex and your connection to the router is a hard-wire not wireless.

http://www.linksys.com/servlet/Satellite?c=L_Product_C2&childpagename=US%2FLayout&cid=1175239516849&pagename=Linksys%2FCommon%2FVisitorWrapper

Can you try swapping the cable and perhaps the port on the router with another machine that you have proven is working?

It seems like a long-shot but what you are experiencing is odd.

If your box is dual boot and the network interface card (NIC) is integrated onto motherboard it might be that Windows somehow lockup the NIC on hardware level and prevent Linux to communicate with it. The solution might be to get a PCI NIC.

you could also post here output of these two commands:

mii-tool
mii-diag

as root. but from the ethtool output it really seems the link (conection) between your computer and the router is established.

trying to connect another computer through that router like Tony Mugan says might be helpful too.

Thanks Scott Scriven, that solved my question.

OMG scott you are a genius! it works perfectly now. Thats weird that it didn't like the default driver. But thinking outside the box really helped me out. I really appreciate your help and everyone else's input as well :-D

Okay, good. One last thing you probably want to know...

To make the change permanent, edit the file /etc/modprobe.conf . One way to do this is by running "sudo nano /etc/modprobe.conf". Keyboard shortcuts are listed onscreen inside nano.

In the file, add a line which says:

  alias eth0 8139too

Then save and quit. This will tell the computer which driver to use in the future.

I have dual boot os win xp and ubuntu 8.4.In wn xp there is no problem on connecting to the internet but when I use ubuntu I can't connect to the internet