Bringup a network interface

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If the interface(for eg: eth0) is not UP automatically on system boot, you can temporarily  bring up the interface like:

# dhclient <eth0>

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To make it permanent, edit /etc/sysconfig/network-scripts/ifcfg-eth0
Set ONBOOT as yes as:
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...
ONBOOT=yes
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Here, eth0 is the interface configured.