Based on http://pachome1.pacific.net.sg/~harish/linuxipalias.html

Please realize that IP aliases and NAT do not play well together, as can be read in this FAQ

GNU Copyleft 1996/1997 Harish Pillay (h.pillay@ieee.org)

This is a cook book recipe on how to set up and run IP aliasing on a FREESCO box.

• setup all the IP #s for the aliases on the eth0 interface:

ifconfig eth0:0 172.16.3.10
ifconfig eth0:1 172.16.3.100

172.16.3.10 and .100 are the aliases. The magic is the eth0:x where x=0,1,2,…n for the different IP #s. The main IP # does not need to be aliased.

• 2nd, setup the routes:

route add -host 172.16.3.10 dev eth0:0
route add -host 172.16.3.100 dev eth0:1

That's it.

In the example IP # above, I am using the Private IP #s (RFC 1918) for illustrative purposes. Substitute them with your own official or private IP #s.

The example shows only 2 IP #s. The max is defined to be 256 in /usr/include/linux/net_alias.h.
256 IP #s on ONE card is a lot !

Here's what my /sbin/ifconfig looks like:

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Bcast:127.255.255.255  Mask:255.0.0.0
          UP BROADCAST LOOPBACK RUNNING  MTU:3584  Metric:1
          RX packets:5088 errors:0 dropped:0 overruns:0
          TX packets:5088 errors:0 dropped:0 overruns:0

eth0      Link encap:10Mbps Ethernet  HWaddr 00:8E:B8:83:19:20
          inet addr:172.16.3.1  Bcast:172.16.3.255  Mask:255.255.255.0
          UP BROADCAST RUNNING PROMISC MULTICAST  MTU:1500  Metric:1
          RX packets:334036 errors:0 dropped:0 overruns:0
          TX packets:11605 errors:0 dropped:0 overruns:0
          Interrupt:7 Base address:0x378 

eth0:0    Link encap:10Mbps Ethernet  HWaddr 00:8E:B8:83:19:20
          inet addr:172.16.3.10  Bcast:172.16.3.255  Mask:255.255.255.0
          UP BROADCAST RUNNING  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0
          TX packets:0 errors:0 dropped:0 overruns:0

eth0:1    Link encap:10Mbps Ethernet  HWaddr 00:8E:B8:83:19:20
          inet addr:172.16.3.100  Bcast:172.16.3.255  Mask:255.255.255.0
          UP BROADCAST RUNNING  MTU:1500  Metric:1
          RX packets:1 errors:0 dropped:0 overruns:0
          TX packets:0 errors:0 dropped:0 overruns:0

And /proc/net/aliases:

device           family address                                
eth0:0           2      172.16.3.10                        
eth0:1           2      172.16.3.100

And /proc/net/alias_types:

type    name            n_attach
2       ip              2   

Of course, the stuff in /proc/net was created by the ifconfig command and not by hand!

Question: How can I keep the settings through a reboot?

Answer: Place the commands in your /rc/rc_user script:

start)
  #setting up IP alias interfaces
  echo "Setting 172.16.3.10, 172.16.3.100 IP Aliases ..."
  ifconfig eth0:0 172.16.3.10
  ifconfig eth0:1 172.16.3.100
  #setting up the routes
  echo "Setting IP routes ..."
  route add -host 172.16.3.10 eth0:0
  route add -host 172.16.3.100 eth0:1
  ;;