If you are using a 2.2 kernel, you need to enable IP aliasing. In a 2.4 kernel this is automatically supported.
If I have a static IP of 192.168.42.1 on eth1, and I wish to create an alias address for that adapter, I do this at the command line:
ifconfig eth1:1 192.168.42.72
A quick check using ifconfig (with no parameters) should show an entry for a device called eth1:1, with address 192.168.42.72.
If it worked, you're ready to add it to /etc/network/interfaces, where ifup and ifdown can get to it.
Code:
auto eth1
iface eth1 inet static
address 192.168.42.1
netmask 255.255.255.0
broadcast 192.168.42.255
network 192.168.42.0
auto eth1:1
iface eth1:1 inet static
address 192.168.42.72
netmask 255.255.255.0
broadcast 192.168.42.255
network 192.168.42.0