segregating nat traffic

[Joe Cave]

question: i'm trying segregate internet <--> lan nat traffic for
different networks using separate nic's for the lan's and a common nic
for the internet (internet nic listens for multiple ip's). i would like
to keep all incoming/outgoing traffic separated so that lan-1 uses
internet_ip-1 and lan-2 uses internet_ip-2 etc...=20

i've done nat like this previously:

iptables -t nat -A POSTROUTING -o <internet nic> -j SNAT --to-source
<internet ip>

but i'm not sure how to completely separate everything by ip/lan while
only having 1 nic for the internet? sorry if this is a dumb question for
this list. the need for this was kinda dropped in my lap suddenly...=20

thanks for any help.

- joe