when use prerouting ???
and when use postrouting ???
thanks 
Hi,
can’t say exactly !
it completely depends on your network configuration.
http://wiki.mikrotik.com/wiki/Manual:Packet_Flow
input : packets coming to the router itself
output : packets going out from the router itself
forward : packets going through the router.
prerouting : before the router routes packets !
postrouting : after the router routes packets based on its routing table
thank you sir