Assuming that we have a webserver hosted by us on our internal network as 192.168.1.1 and we map it to pix outside interface ip address x.x.x.x as follows:
static (inside,outside) x.x.x.x 192.168.1.1 netmask 255.255.255.255 then any traffic from outside world hitting to x.x.x.x will directly be routed to 192.168.1.1. Hence, we use port forwarding as follows:static (inside,outside) tcp interface 80 192.168.1.1 80 so that only the port 80 traffic destined to x.x.x.x should be forwarded to 192.168.1.1 and not all the traffic. And you can also do by change the ISP cable to your device and reconfigure it .