What is the good practice for the placement of IDS (NIDS) and IPS? Before the firewall or after the firewall? and why?
The answer is: it depends.
If you need to see all the crap hitting you from the Internet, IDS should be outside the firewall. Although it is not recommended because you literally get flooded with useless data and gives the machine lots of load. If not, just put it in.
IPS should be inside the firewall because it consumes more time per packet to inspect than the firewall.