about prerouting and postrouting

[veeraafed]

dear all,
Colud any body please tell me what is pre-routing and post-routing and how its working and if its possible could you please tell me with example,
|---------|
-- eth0 |
| |
-- eht1 |
|---------|

where eth0 is publicip
where eth1 is privateip

thanks and regards

bose

[Jman]

Try looking through iptables guides