/etc/sysconfig/network-scripts/ifcfg-eth0

This file contains the configuration of your network card. The settings are different for dhcp connection and for a static connection.

For a dhcp connection the file looks like

# Advanced Micro Devices [AMD] 79c970 [PCnet32 LANCE]
DEVICE=eth0
BOOTPROTO=dhcp
HWADDR=00:0c:29:d6:04:92
ONBOOT=yes
TYPE=Ethernet

For static connection it looks like

# Advanced Micro Devices [AMD] 79c970 [PCnet32 LANCE]
DEVICE=eth0
BOOTPROTO=none
HWADDR=00:0c:29:d6:04:92
ONBOOT=yes
TYPE=Ethernet
NETMASK=255.255.255.0
IPADDR=192.168.1.3
GATEWAY=192.168.1.1