RHEL/CentOS 6/7 does not configure network interfaces on default installation. Also, it is configured to use NetworkManager. To enable networking interface and disable NetworkManager please follow instructions below.

The /etc/sysconfig/network-scripts/ifcfg-eth0 content looks as follows on default installation.

DEVICE="eth0"
HWADDR=specifc mac address
NM_CONTROLLED="yes"
ONBOOT="no"

For DHCP, configure the interface as follows.

DEVICE="eth0"
HWADDR=specifc mac address
NM_CONTROLLED="no"
ONBOOT="yes"
BOOTPROTO="dhcp"

 Restart the network after configuration.
chkconfig NetworkManager off
service NetworkManager stop
/etc/init.d/network restart

For Static ip, configure it as follows

DEVICE="eth0"
HWADDR=specifc mac address
NM_CONTROLLED="no"
ONBOOT="yes"
BOOTPROTO="static"
IPADDR=ip address here.
NETMASK=netmask here.

Change the /etc/sysconfig/network to have the GATEWAY.

NETWORKING=yes
HOSTNAME=www1.nixcraft.in
GATEWAY=gateway address here.

Change /etc/resolve.conf
nameserver Specify ip address of dns server/usually same as gateway.

For example configuration, see  Method #3 
http://www.cyberciti.biz/faq/rhel-centos-fedoracore-linux-network-card-configuration/

Restart the network after configuration.
chkconfig NetworkManager off
service NetworkManager stop
/etc/init.d/network restart