To permanently add one or more IP's to network interface in RedHat Linux is quite easy if you follow this steps:

• Go to the /etc/sysconfig/network-scripts/ directory
  

  cd /etc/sysconfig/network-scripts/

• Copy the interface file that you want to add the IP to, and give it the same name as the current file, with ‘:0′ added (or :1, :2, depending on how many IPs are on this interface).
  

  cp ifcfg-eth0 ifcfg-eth0:0

• Edit the new file, and you only have to change a few things
  

  > Change IPADDR= to your new IP address
  > Change DEVICE= to add :0
  (depending if this is the 2nd IP on that interface)
  > Remove DHCP_HOSTNAME (or set it to what it needs to be)
  > remove the HWADDR entry

• Save the file that you just were working on
• Restart the network interfaces
  

  /etc/init.d/network restart

• Done!

So here are the examples:

Original file ifcfg-eth0 looks like

# Intel Corporation 82557/8/9 [Ethernet Pro 100]
DEVICE=eth0
BOOTPROTO=none
BROADCAST=192.168.5.255
HWADDR=00:06:5B:38:C8:5D
IPADDR=192.168.5.230
NETMASK=255.255.255.0
NETWORK=192.168.5.0
ONBOOT=yes
GATEWAY=192.168.5.1
TYPE=Ethernet
USERCTL=no
IPV6INIT=no
PEERDNS=yes

Config file ifcfg-eth0:0 for second IP looks like:

DEVICE=eth0:0
BOOTPROTO=none
BROADCAST=192.168.5.255
IPADDR=192.168.5.20
NETMASK=255.255.255.0
NETWORK=192.168.5.0
GATEWAY=192.168.5.1
TYPE=Ethernet
USERCTL=no
IPV6INIT=no
PEERDNS=yes
ONPARENT=yes

nameserver 192.168.1.1
nameserver 10.0.0.138

127.0.0.1 localhost.localdomain localhost
194.109.6.67 ns.xs4all.nl

/etc/init.d/network restart

If you are connected with SSH be patient connection will be restored.