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As per subject, I have a Zimbra mail server on Centos 5.5. and the problem is there that when I plug the LAN cable in to the server port all Internet and network services go down.

This means the network is not working and ping also does not work. When I unplug the cable from the server's LAN port everything is OK.

Can anybody guide me as to what I could do in this situation.

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Do you mean that your network is working perfectly, but then when you connect your Centos machine all networking activity breaks, even among the other machines on your network? – Nate from Kalamazoo Jun 26 '13 at 12:28
If the Internet connectivity fails only on that particular host when you connect the network cable, please edit your question to include the output of route -n executed on that host before and after connecting the network cable. – Michael Kjörling Jun 26 '13 at 12:57

Perhaps your Centos machine has a statically set IP address that conflicts with some other critical piece of hardware, such as your gateway or router?

You could troubleshoot this as follows:

On some other Linux machine that's capable of accessing the internet, check to see what its default route is:

sudo route |grep -E '(Gateway|default)'

The line of output which begins with default should list an IP address immediately following the word default - this is probably the IP address of your gateway or router.

Check to make sure that your Centos machine isn't configured to use the same address by connecting it to the network and then checking its IP address(es) with

ifconfig -a |grep -B1 'inet addr'

If any of those addresses match that of your router, that is likely the source of your problem. Change the IP address of your server to something else like

ifconfig eth0 down && ifconfig eth0 netmask up

If that works, you can make that IP address change permanent by editing /etc/sysconfig/network-scripts/ifcfg-eth0

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