multiple outbound NAT

[Bonno Bloksma <b.bloksma@tio.nl>]

Hi,

At one place I have a Debian wheezy machine that acts as router / firewall using iptables and default routing.
I used to have just 1 ip number on the uplink interface. And a simple 
  $IPTABLES --table nat -A POSTROUTING -o $WORLD_IF -j MASQUERADE
line in my firewall script sends all traffic out with that single ip addres via NAT.

Due to several reasons I now have to use more than 1 outbound ip address to make clear from which internal segment the traffic is coming from.
So traffic coming from 172.16.20.0/24 needs to use $WORLD_IP1
And traffic coming from 172.16.22.0/24 needs to use $WORLD_IP2
And maybe traffic coming from 172.16.24.0/23 needs to use $WORLD_IP3

How do I configure something like that?
If it is any use in this case, I have quagga on that machine as well but so far that is used to do routing for the internal network. But I'd rather not mess with that config.

Bonno Bloksma