Configuring multiple IP addresses on one network interface is commonly called IP Aliasing. It is very useful when you setup multiple sites on virtual web hosting on a single interface.

There are few ways to setup IP Aliasing. One is using ifconfig like:

[root@burnz ~]# ifconfig eth0:1 192.168.16.19 netmask 255.255.255.248 up
[root@burnz ~]# ifconfig eth0:2 192.168.16.20 netmask 255.255.255.248 up

To verify all ip addresses assigned to eth0 run this command:

[root@burnz ~]# ip addr list dev eth0

Results would be:

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast qlen 1000
    link/ether 00:19:f6:d8:c8:86 brd ff:ff:ff:ff:ff:ff
    inet 192.168.166.18/29 brd 192.168.16.23 scope global eth0
    inet 192.168.16.19/29 brd 192.168.16.23 scope global secondary eth0:1
    inet 192.168.16.20/29 brd 192.168.16.23 scope global secondary eth0:2
    inet6 fe80::246:e3gg:feb8:f591/64 scope link
       valid_lft forever preferred_lft forever

You can also use ifconfig command to verify:

[root@burnz ~]# ifconfig -a
eth0      Link encap:Ethernet  HWaddr 00:19:F6:D8:C8:86
          inet addr:192.168.16.18  Bcast:192.168.16.233  Mask:255.255.255.248
          inet6 addr: fe80::246:e3gg:feb8:f591/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:11333116 errors:0 dropped:0 overruns:0 frame:0
          TX packets:9978256 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:1427419683 (1.3 GiB)  TX bytes:1562204398 (1.4 GiB)

eth0:1    Link encap:Ethernet  HWaddr 00:19:F6:D8:C8:86
          inet addr:192.168.16.19  Bcast:192.168.16.23  Mask:255.255.255.248
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1

eth0:2    Link encap:Ethernet  HWaddr 00:19:F3:D9:C8:86
          inet addr:192.168.16.20  Bcast:192.168.16.23  Mask:255.255.255.248
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1

eth1      Link encap:Ethernet  HWaddr 00:19:F3:DB:C3:BB 
          inet addr:172.2.8.194  Bcast:172.2.8.223  Mask:255.255.255.224
          inet6 addr: fe80::216:e3ff:feab:f3bb/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:124270 errors:0 dropped:0 overruns:0 frame:0
          TX packets:60627 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:8176024 (7.7 MiB)  TX bytes:5185410 (4.9 MiB)

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:628 errors:0 dropped:0 overruns:0 frame:0
          TX packets:628 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:36416 (35.5 KiB)  TX bytes:36416 (35.5 KiB)

You can also use a manual way by modifying the network config file:

[root@burnz ~]# nano /etc/sysconfig/network-scripts/ifcfg-eth0:1

DEVICE=eth0:1
BOOTPROTO=static
IPADDR=192.168.16.19
NETMASK=255.255.255.248
ONBOOT=yes

$ nano /etc/sysconfig/network-scripts/ifcfg-eth0:2

DEVICE=eth0:2
BOOTPROTO=static
IPADDR=192.168.16.20
NETMASK=255.255.255.248
ONBOOT=yes

Once created you  need to restart the network to activate the IP Aliasing:

[root@burnz ~]# /etc/init.d/network restart

It should be good!