http://www.debian.org/doc/manuals/debian-reference/ch05.en.html#%5Fthe%5Fvirtual%5Finterface

A simple example, assuming your primary network interface is

eth0

and has an ip of

192.168.1.1

and the new ip is

192.168.1.2

:

auto eth0
iface eth0 inet static
  address 192.168.1.1
  netmask 255.255.255.0
  gateway 192.168.1.254

auto eth0:0
iface eth0:0 inet static
  address 192.168.1.2
  netmask 255.255.255.0

Once the appropriate settings have been added to

/etc/network/interfaces

, run

ifup eth0:0

to activate the new interface.

admin