Use of /etc/sysconfig/network-scripts/ifcfg-eth* files

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http://unix.stackexchange.com – What's the use of /etc/sysconfig/network-scripts/ifcfg-eth* files ? In some cases ifconfig lists the interface eth0 but I don't find the corresponding ifcfg-eth0 file whereas in other instances if I create the ifcfg-eth1 file and restart the network like service network restart - this interface is not listed in ifconfig ?? (HowTos)