Start network after boot on CentOS

You have two options, use the network-scripts or networkmanager

For myself, I want to use the network scripts. 

chkconfig NetworkManager off
chkconfig network on

Disable the NetworkManager and enable the old network

Then, you need to make sure you are setting up a correct network script at /etc/sysconfig/network-scripts/ifcfg-eth0

make sure ONBOOT=yes

Example of external network interface using DHCP:

$ vim /etc/sysconfig/network-scripts/ifcfg-em1

DEVICE=em1

HWADDR=00:26:B9:3E:33:A9

TYPE=Ethernet

ONBOOT=yes

NM_CONTROLLED=no

BOOTPROTO=dhcp

DHCP_HOSTNAME=sr2s1

PEERDNS=yes

Example of internal network interface using static IP:

vim /etc/sysconfig/network-scripts/ifcfg-p1p1

DEVICE=p1p1

HWADDR=00:1B:21:44:8D:70

TYPE=Ethernet

ONBOOT=yes

NM_CONTROLLED=no

BOOTPROTO=static

IPADDR=192.168.0.21

BROADCAST=192.168.0.255

NETMASK=255.255.255.0

NAME=p1p1