there are many ways, first using deprecated ifconfig (ifconfig will be replaced by ip) :

======================
[root@h4x0rb0x network-scripts]# ifconfig -a
eth0      Link encap:Ethernet  HWaddr 00:30:1B:40:AC:DF 
          inet addr:**********  Bcast:************  Mask:255.255.255.0
          inet6 addr: **********/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:131808 errors:0 dropped:0 overruns:0 frame:0
          TX packets:80433 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:23982501 (22.8 MiB)  TX bytes:11595617 (11.0 MiB)
          Interrupt:217 Base address:0xc000

eth0:0    Link encap:Ethernet  HWaddr 00:30:1B:40:AC:DF 
          inet addr:************  Bcast:***********  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          Interrupt:217 Base address:0xc000

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:46631 errors:0 dropped:0 overruns:0 frame:0
          TX packets:46631 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:10203334 (9.7 MiB)  TX bytes:10203334 (9.7 MiB)

sit0      Link encap:IPv6-in-IPv4 
          NOARP  MTU:1480  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:0 (0.0 b)  TX bytes:0 (0.0 b)
===========

where eth0:1 is an alias of eth0 :
============
[root@h4x0rb0x]# ifconfig eth0:1 new_allocated_ip netmask 255.255.255.0
[root@h4x0rb0x]#  ifconfig eth0:1   ip
==============


second way via network scripts:
========================
[root@h4x0rb0x network-scripts]# cat /etc/sysconfig/network-scripts/ifcfg-eth0:1
IPADDR=new_allocated_ip
NETMASK=255.255.255.0
DEVICE=eth0
ONBOOT=yes
BOOTPROTO=none
TYPE=Ethernet
ETHTOOL_OPTS="duplex full speed 100 autoneg off wol g"
[root@h4x0rb0x network-scripts]#