Linux Poison posted a guide about configuring IP aliases under Ubuntu Linux

IP aliasing is the process of adding more than one IP address to a network interface. Typical uses of IP aliasing are virtual hosting of Web and FTP servers.  It is fairly simple to set up an IP alias. You merely have to configure the network interface on the system to listen for the additional IP address.

Below are the steps for configuring IP alias under Ubuntu Linux:
If you want to put IP alias on an interface, first find the interface, use command

ifconfig

  Configuring IP aliases under Ubuntu Linux