I recently worked on a difficult connection problem on my father's computer. This post will share some of the problems and the solution in hopes of helping others solve the problem more quickly than I did. Our relevant system specs are listed at the end of the post.
The Problem: Suddenly the computer was not able to connect to the Internet (including email).
The Cause: AT&T stopped honoring the default usercode/password that was stored in the DSL modem, with no warning and no meaningful error messages. The connection simply stopped working.
The Solution: AT&T support reset something giving me temporary access. I went to an AT&T login page where I was able to change the usercode and password. I reset the modem by using a paperclip to press the reset button in the hole in the back of the modem. Then I was able to enter and save the new usercode and password in the DSL modem.
I'll share some symptoms, actions I performed, and pitfalls so you can see if your problem is similar to ours.
No Internet connection and no email connection.
The modem lights indicated everything was fine. There were four solid green lights and the fifth light blinked green occasionally. None of the lights were red or yellow.
Neighbors who were also BellSouth DSL subscribers did not have a problem.
Attempting to connect to the Internet using Internet Explorer resulted in the error message, "Internet Explorer cannot display the webpage." Clicking on "Diagnose Connection Problems" on the error screen resulted in the following error window.
Windows Network Diagnostics
Windows confirmed that "www.google.com" is currently online, but is not responding to connection attempts at this time.
This usually means that a firewall is running somewhere between the two computers and is blocking "World Wide Web service (HTTP)". Windows has confirmed that Windows Firewall on this computer is correctly configured to allow this connection. However, a remote firewall might be blocking your connection.
If you have access to this firewall then configure the firewall to allow connections through TCP port 80. If you don't have access, contact your network administrator or Internet service provider.
Windows Network Diagnostics confirmed that each address we tried was online but not responding, indicating that there was a physical connection to the Internet. Microsoft even automatically downloaded and installed a Windows update, further proving the connection existed and was working, but was being blocked when we tried to use it. (Diagnostics can also be run from the pop-up menu you get when you right-click on the network icon on the lower right in the system tray.)
Attempting to download email also caused an error.
Reset the network adapter using a link in the Windows Network Diagnostics results. No effect.
Checked wires. All were intact.
Rebooting the computer and rebooting the modem by disconnecting and reconnecting the power cable had no effect.
I accessed the modem's user interface using the IP address listed in the manual. I ran the modem's full diagnostics. All tests passed.
Turning off the Windows Domain, Public, and Private firewalls had no effect.
Disabling and later uninstalling Avast! antivirus had no effect.
Stopping LogMeIn and later uninstalling LogMeIn had no effect.
Uninstalling Windows Vista Service Pack 1, which had installed the previous day, had no effect.
Restoring the system to a restore point created while the computer still had Internet access had no effect.
Downloading and installing another driver for the network adapter did not help.
Disabling and enabling the network adapter did not help.
I called AT&T support and ran several commands the technician walked me through. We opened a DOS window by clicking Start on the lower left and then entering "CMD" in the search field. I ran "ipconfig /release" which resulted in an error saying elevation was required. This put off the technician. (I was running Windows under an administrative account.) I ran "ipconfig /renew" which didn't help. I ran "ipconfig /all" and read the technician the results. When I got to the result that said something like, "Tunnel Adapter Local Area Connection *6 Teredo Tunneling Pseudo-Interface Media Disconnected", the technician said she thought I had either a bad network adapter or a bad Ethernet cable. Since she thought the problem was with the hardware, she said she couldn't be of any more assistance, although she did give me a fee-based AT&T technical support number I could call. (Since then I have learned that several of the connections listed on the "ipconfig /all" results say "Media Disconnected" even when the online connection is working fine.)
I knew the adapter and LAN cable were good since Microsoft was able to download updates, but I realized the best way to make AT&T continue working on the problem would be to prove the adapter and cable were not the problem. So we bought a new Netgear GA311 network adapter and a new LAN cable. We installed those and still had no Internet access.
I called AT&T support again, told the technician we had replaced the parts the previous technician had said were defective, and we still had no Internet access. He determined that the usercode/password stored in the modem were old default values that AT&T had deactivated. He reset something giving us temporary access. He gave us a url to visit where I changed the usercode and password. Then I accessed the modem's user interface and entered the new usercode and password. Problem solved.
Relevant Computer Specs.
Internet Service: BellSouth (AT&T) FastAccess DSL
Operating System: Windows Vista Home Premium
Browser: Internet Explorer 7
DSL Modem: Motorola Netopia 2210-02
Internet Adapter: Intel 82562V 10/100 Network Connection (on motherboard)
Netgear GA311 network adapter (added)