Ethernet will not work

Question

I am connecting through a sonic wall router to my iMac. My airport runs great -- I have never used the ethernet. Now I am in a place where I can use the ethernet. I connect the cord, and the ethernet connects then disconnects, then connects, disconnects etc. I replaced the cord thinking it was bad. Same thing happened. So I brought my MBP and tried connecting the ethernet and it worked fine. I called Apple (I am still under warranty) and "Dave" spent over an hour on the phone with me having me do all kinds of things. Now, he wants me to reformat my hard drive -- I thought only Dell told you to do that when all else fails!
Any chance this is really, as I am thinking, a hardware issue rather than a software issue?
Oh, when I pinged it from the terminal, I got 38% packet loss. when I did the same with the mbp, there was 0% loss.

Answer 1

Don't know if you've done this. First, open Network preferences, select the Ethernet port and click on the Advanced button. Click on the TCP/IP tab. Set the IPv4 drop down menu to DHCP and set the IPv6 drop down menu to OFF. Click on the Apply button. Now, there's a tool drop down menu just below the port list. Select Set Service Order, then move the Ethernet port to the top of the list, place the Airport second. Click on the OK button then click on the Apply button.
Now shut down the computer and remove power from your cable/DSL modem and router. Wait not less than one minute. First, turn on the cable/DSL modem and wait for the status lights to indicate connection to the ISP. Next, turn on the router and wait for the status light to indicate connection with the modem. Last, turn on the computer.
If this doesn't help then try reinstalling OS X. You may not need to reformat your drive. See the following:
How to Perform an Archive and Install
An Archive and Install will NOT erase your hard drive, but you must have sufficient free space for a second OS X installation which could be from 3-9 GBs depending upon the version of OS X and selected installation options. The free space requirement is over and above normal free space requirements which should be at least 6-10 GBs. Read all the linked references carefully before proceeding.
1. Be sure to use Disk Utility first to repair the disk before performing the Archive and Install.
Repairing the Hard Drive and Permissions
Boot from your OS X Installer disc. After the installer loads select your language and click on the Continue button. When the menu bar appears select Disk Utility from the Installer menu (Utilities menu for Tiger.) After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list. In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive. If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported, then quit DU and return to the installer.
2. Do not proceed with an Archive and Install if DU reports errors it cannot fix. In that case use Disk Warrior and/or TechTool Pro to repair the hard drive. If neither can repair the drive, then you will have to erase the drive and reinstall from scratch.
3. Boot from your OS X Installer disc. After the installer loads select your language and click on the Continue button. When you reach the screen to select a destination drive click once on the destination drive then click on the Option button. Select the Archive and Install option. You have an option to preserve users and network preferences. Only select this option if you are sure you have no corrupted files in your user accounts. Otherwise leave this option unchecked. Click on the OK button and continue with the OS X Installation.
4. Upon completion of the Archive and Install you will have a Previous System Folder in the root directory. You should retain the PSF until you are sure you do not need to manually transfer any items from the PSF to your newly installed system.
5. After moving any items you want to keep from the PSF you should delete it. You can back it up if you prefer, but you must delete it from the hard drive.
6. You can now download a Combo Updater directly from Apple's download site to update your new system to the desired version as well as install any security or other updates. You can also do this using Software Update.