To add a secondary IP to CentOSnavigate to your /etc/sysconfig/network-scripts

# cd /etc/sysconfig/network-scripts

[/bash]

# ls -la
-rw-r--r--. 1 root root   203 Dec 30 14:18 ifcfg-eth0
-rw-r--r--. 1 root root   230 Dec 30 14:59 ifcfg-eth1

The system has a public and primary IP.

# nano ifcfg-eth0

[/bash]

DEVICE=eth0
HWADDR=00:0C:29:E7:7D:05
TYPE=Ethernet
UUID=b9cbc9ab-d4a5-4ca8-9e3c-176c62102adc
ONBOOT=yes
NM_CONTROLLED=no
BOOTPROTO=static
IPADDR=216.55.000.000
GATEWAY=216.55.169.1
NETMASK=255.255.255.0

To add a public IP, create new files with ifcfg-eth0:1, ifcfg-eth0:2, etc