As with other operating systems it is possible to bring multiple service ip-addresses online under one physical NIC under linux. This is just at brief howto on doing it.
ubuntu:
sudo vi /etc/network/interfaces
# The primary network interface
auto eth0
iface eth0 inet static
address 192.168.1.2
netmask 255.255.255.0
network 192.168.1.0
broadcast 192.168.1.255
gateway 192.168.1.1
# The first virtual interface
auto eth0:1
iface eth0:1 inet static
 address 192.168.1.100
 netmask 255.255.255.0
 broadcast 192.168.1.255
gateway 192.168.1.1
After you have edited that file, issue
sudo /etc/init.d/networking restart
RHEL (Redhat)/Centos:
cd /etc/sysconfig/network-scripts
sudo cp ifcfg-eth0 ifcfg-eth0:0
sudo vi ifcfg-eth0:0
cat ifcfg-eth0:0
> DEVICE=eth0:0
> BOOTPROTO=none
> IPADDR=192.168.1.100
>Â NETMASK=255.255.255.0
> GATEWAY=192.168.100
> ONBOOT=yes
After that, issue
sudo service network restart